materials
Article
An Approach to Solid-State Electrical Double Layer
Capacitors Fabricated with Graphene Oxide-Doped,
Ionic Liquid-Based Solid Copolymer Electrolytes
N. F. A. Fattah, H. M. Ng, Y. K. Mahipal, Arshid Numan, S. Ramesh * and K. Ramesh
Centre for Ionics Universiti Malaya, Department of Physics, Faculty of Science, University of Malaya,
Kuala Lumpur 50603, Malaysia; faiqahfattah@gmail.com (N.F.A.F.); nghonming@hotmail.com (H.M.N.);
ykmahipal@gmail.com (Y.K.M.); numan.arshed@yahoo.com (A.N.); ramesh@um.edu.my (K.R.)
* Correspondence: rameshtsubra@gmail.com; Tel.: +603-79674391
Academic Editor: Douglas Ivey
Received: 25 March 2016; Accepted: 30 May 2016; Published: 6 June 2016
Abstract: Solid polymer electrolyte (SPE) composed of semi-crystalline poly (vinylidene
fluoride-hexafluoropropylene) [P(VdF-HFP)] copolymer, 1-ethyl-3-methylimidazolium bis
(trifluoromethyl sulphonyl) imide [EMI-BTI] and graphene oxide (GO) was prepared and its
performance evaluated. The effects of GO nano-filler were investigated in terms of enhancement
in ionic conductivity along with the electrochemical properties of its electrical double layer
capacitors (EDLC). The GO-doped SPE shows improvement in ionic conductivity compared to
the P(VdF-HFP)-[EMI-BTI] SPE system due to the existence of the abundant oxygen-containing
functional group in GO that assists in the improvement of the ion mobility in the polymer
matrix. The complexation of the materials in the SPE is confirmed in X-ray diffraction (XRD) and
thermogravimetric analysis (TGA) studies. The electrochemical performance of EDLC fabricated
with GO-doped SPE is examined using cyclic voltammetry and charge–discharge techniques. The
maximum specific capacitance obtained is 29.6 F¨g´1 , which is observed at a scan rate of 3 mV/s
in 6 wt % GO-doped, SPE-based EDLC. It also has excellent cyclic retention as it is able keep the
performance of the EDLC at 94% even after 3000 cycles. These results suggest GO doped SPE plays a
significant role in energy storage application.
Keywords: copolymer; graphene oxide; electrical double layer capacitor; ionic liquid
1. Introduction
Electronic devices are becoming essential belongings for people around the world in their daily
life. It is something that is carried along almost everywhere at all times. Thus, the need for these
devices to be lightweight, thin and safe is extremely high. The conventional liquid electrolyte batteries
used in these electronic devices now are mostly bulky and heavy and could cause safety issues due to
the risk of harmful liquid leakages [1]. Recent research has shown tremendous advancements in an
alternative to liquid electrolyte batteries: solid polymer electrolytes (SPE). Their ability to form thin
films, light weight, flexibility and reasonable ionic conductivity are among the useful benefits of using
solid polymer electrolyte [2].
In previous researches, many different polymers have been used in the application of SPE and
most of them are conventional homopolymers such as PVC and PAN [3]. In more recent work, the
copolymer has emerged as a potent choice to be used as the host polymer due to the advantages it
could provide. For example, in this work, P(VdF-HFP) is used as the host polymer. This copolymer is
the result of the copolymerization between poly (vinylidene fluoride) [PVdF] and hexafluoropropylene
[HFP]. PVdF has high crystallinity while HFP has high amorphous phase [4]. When copolymerized,
Materials 2016, 9, 450; doi:10.3390/ma9060450
www.mdpi.com/journal/materials
Materials 2016, 9, 450
2 of 15
the HFP region could drastically reduce the crystallinity and promote more amorphous regions within
the copolymer, a desirable quality, which is needed in polymer electrolytes [5]. HFP phase provides
higher ionic conduction and a PVdF region provides mechanical stability for the polymer electrolyte
film [6].
In this research, 1-ethyl-3-methylimidazolium bis (trifluoromethyl sulphonyl) imide [EMI-BTI]
ionic liquid is used. Ionic liquids are molten salts that are in liquid state at room temperature. Ionic
liquid was proven to have the ability to enhance the ionic conductivity of the polymer electrolyte due
to its high electrical performance properties [7]. In a solid polymer electrolyte system which has no
salt added, ionic liquid plays a huge role in providing the mobile ions to allow ion conduction to occur
within the polymer matrix of the SPE. Besides having good electrical properties, ionic liquid also well
known to have low combustibility as well as excellent chemical and thermal stability which are very
much important for the application of SPEs [8].
In addition, nanofiller is usually added into the polymer electrolytes for the purpose of
improving the properties of the polymer electrolytes in terms of electrochemical performances, thermal
properties, etc. [9]. In this research, we focused on the effect of the addition of different weight
percentages of graphene oxide (GO), an inorganic nanofiller into the solid polymer electrolyte. GO has
a unique property in that it has multiple oxygen-containing functional groups which could enable it to
be well dispersed in water or polar solvents and could also improve the mobility of the mobile ions
and lead to the improvement of the ion conductions of the polymer electrolytes [10,11].
The GO-doped solid polymer electrolytes were then used with activated carbon electrodes to
fabricate EDLC. EDLC has advantages over the usual rechargeable batteries available. It has higher
charge energy density, higher rate of charge-discharge and longer cycle performance compared to
conventional rechargeable batteries [12–15]. The electrical charges are stored based on electrostatic
interaction in the electric double layer at the interface of electrode or electrolyte [16]. Activated carbon
is commonly used as the negative and positive electrodes while the solid polymer films are used as the
electrolyte and separator between the cathode and anode. Activated carbon has the characteristics
of cost effective, high porosity and high surface area [17]. This project has investigated the electrical
performance of GO-doped P(VdF-HFP) solid polymer electrolyte-based EDLC in order to learn more
about the compatibility of our SPE for the EDLC application.
2. Methodology
2.1. Materials
Poly (vinylidene fluoride-hexafluoropropylene) [P(VdF-HFP)], 1-Ethyl-3-methylimidazolium
bis (trifluoromethyl sulphonyl) imide [EMI-BTI] and acetone were obtained from Sigma-Aldrich
(St. Louis, MO, USA). Meanwhile, graphene oxide (GO) was synthesized with the method according
to a study reported by See et al. [14]. The sulphuric acid [H2 SO4 ], potassium permanganate [KMnO4 ]
and hydrogen peroxide [H2 O2 ] that were used for the synthesis of graphene oxide were obtained from
Sigma-Aldrich as well.
2.2. Synthesis of Graphene Oxide
Graphene oxide was prepared using simplified Hummer’s method from graphite. 400 mL of
H2 SO4 and 18 g of KMnO4 were used to treat 3 g of graphite flakes oxidatively. The mixture was then
stirred for 5 min for the graphite oxide to be formed. In order to secure complete oxidation of graphite,
the mixture was then stirred for an additional 3 days. Colour changed during the oxidation process can
be observed during the whole stirring process. After that, the change of colour of the mixture to bright
yellow after the addition of H2 O2 solution showed that highly oxidized graphite had been obtained.
The graphite oxide was then rinsed 3 times with 1 M of HCL and followed by 6-times deionized water
to obtained GO gel [18].
Materials 2016, 9, 450
3 of 15
2.3. Preparation of Graphene Oxide Doped Solid Polymer Electrolyte
P(VdF-HFP)-[EMI-BTI]-based SPE was prepared with a different ratio of the P(VdF-HFP) and
EMI-BTI
as 9,
seen
mL
Materials 2016,
450 in Table 1. Respective weight P(VdF-HFP) and EMI-BTI was added into 103 of
15
˝
of acetone in a small container and the mixture was then stirred for 2 h at 40 C for homogeneity.
Then,
the homogeneous
ontopetri-dish.
a glass petri-dish.
Thewas
mixture
was
then
the homogeneous
viscousviscous
mixturemixture
was castwas
ontocast
a glass
The mixture
then left
heated
˝ C in the oven for 12 h for the thin films to be formed. The sample with the
left
at 80
at 80heated
°C in the
oven
for 12 h for the thin films to be formed. The sample with the highest optimum
highest
optimum
conducting
(OCC)
was
then added
with
graphene
oxide. A series
conducting
composition
(OCC)composition
was then added
with
graphene
oxide.
A series
of P(VdF-HFP)-[EMIof
P(VdF-HFP)-[EMI-BTI]-GO
SPEs
(2 wt
% to
8 wt
% of GO) with
theincomposition
seen in Table
1
BTI]-GO
SPEs (2 wt % to 8 wt %
of GO)
with
the
composition
as seen
Table 1 wereasprepared
by the
were
prepared
by
the
same
solution
casting
method
as
mentioned
above.
Figure
1
shows
the
chemical
same solution casting method as mentioned above. Figure 1 shows the chemical structure of the used
structure
materials.of the used materials.
Figure 1. Chemical structure of (a) P(VdF-HFP); (b) EMI-BTI and (c) proposed GO.
Figure 1. Chemical structure of (a) P(VdF-HFP); (b) EMI-BTI and (c) proposed GO.
Table 1. The composition of PVdF-HFP-[EMI-BTI]-based SPE samples.
Table 1. The composition of PVdF-HFP-[EMI-BTI]-based SPE samples.
Electrolytes Composition PVdF-HFP:EMI-BTI:GO PVdF-HFP (g) EMI-BTI (g) GO (g)
Electrolytes
Composition PVdF-HFP:EMI-BTI:GO
PVdF-HFP (g)
SPE10
90:10:0
0.900 EMI-BTI (g) 0.100 GO (g) SPE10
90:10:0
0.900 0.800
0.100
SPE20
80:20:0
0.200
SPE20
80:20:0
0.800
0.200
SPE30
70:30:0
0.300
SPE30
70:30:0
0.700 0.700
0.300
SPE40
60:40:0
0.600 0.600
0.400
SPE40
60:40:0
0.400
GO2
68.6:29.4:2
0.686
0.294
0.02
GO2
68.6:29.4:2
0.686
0.294 0.04 0.02
GO4
67.2:28.8:4
0.672
0.288
GO4
67.2:28.8:4
0.288 0.06 0.04
GO6
65.8:28.2:6
0.658 0.672
0.282
GO8
64.4:27.6:8
0.644 0.658
0.276
GO6
65.8:28.2:6
0.282 0.08 0.06
GO8
64.4:27.6:8
0.644
0.276
0.08
2.4. Fabrication of the Electrode
2.4. Fabrication
of the
Electrode
The electrode
was
fabricated with ratio of 75 wt % activated carbon with a particle size between
5–20 The
µm and
surface
area
betweenwith
1800ratio
andof
2000
m2%
/g,
10 wt %carbon
Poly (vinylidene
fluoride)
(PVdF)
electrode
was
fabricated
75 wt
activated
with a particle
size between
2
binder
and
15
wt
%
carbon
black
(Super
P).
This
mixture
was
mixed
in
1-Methyl-2-pyrrolidone
(NMP)
5–20 µm and surface area between 1800 and 2000 m /g, 10 wt % Poly (vinylidene fluoride) (PVdF)
solvent
to produce
a slurry
The slurry
using
drop casting
method
was coated onto 1 cm2
binder and
15 wt %
carbonmixture.
black (Super
P). This
mixture
was mixed
in 1-Methyl-2-pyrrolidone
stainless
steel foil.
The foil was
rubbed
with different
grades
of drop
emerycasting
paper method
rinsed with
(NMP) solvent
to produce
a slurry
mixture.
The slurry
using
washydrochloric
coated onto
2
acid
(HCl),
deionized
water
and
immersed
in
acetone
before
coating.
The
electrodes
were
dried
in
1 cm stainless steel foil. The foil was rubbed with different grades of emery paper rinsed
with
hydrochloric acid (HCl), deionized water and immersed in acetone before coating. The electrodes
were dried in an oven at 100 °C overnight. The difference of mass of foil before and after coating was
calculated in order to obtain the mass of loading of carbon mixture [19].
Materials 2016, 9, 450
4 of 15
an oven at 100 ˝ C overnight. The difference of mass of foil before and after coating was calculated in
order to obtain the mass of loading of carbon mixture [19].
2.5. Characterization Techniques
The solid polymer electrolyte samples were subjected to EIS characterization using
Hioki 3532-50 LCR-Hi tester (Nagano, Japan) in the frequency range of 50–5,000,000 Hz at room
temperature. The ionic conductivities σ (S¨cm´1 ) of all the samples were then calculated by using the
following equation [20]:
σ “ t{pRb ˆ Aq
(1)
where Rb is the bulk resistance of the sample which was obtained from the Nyquist impedance plot,
A is the area of the contact of the electrodes with the electrolytes in cm2 and t is the thickness of the
sample in cm. Temperature-dependent ionic conductivity was measured at temperatures that varied
from 30 ˝ C to 60 ˝ C.
FTIR studies of all the SPE samples were conducted using a Thermoscientific Nicolet iS10 FTIR
spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) over the range of 650–4000 cm´1 . A
Philips X-ray diffractometer with Cu Kα radiation (PANalytical, Almelo, The Netherlands) was
used to identify the crystallinity and amorphous phases of the samples. The 2θ angle is varied
from 0˝ to 55˝ . Thermal properties were studied using thermogravimetric analysis TGA Q500 V20.13
Build 39 (TA, Instruments, New Castle, DE, USA) with a temperature up to 800 ˝ C.
2.6. Fabrication and the Electrochemical Measurements of EDLC
The EDLC cells were fabricated using the configuration: AC/GO-doped SPE/AC. The cells
were then characterized using cyclic voltammetry (CV), galvanostatic charge-discharge (CCD) and
electrochemical impedance spectroscopy (EIS) with AC amplitude of 10 mV from 0.01 to 100,000 Hz
frequency range.
3. Results and Discussion
3.1. EIS Studies of Solid Polymer Electrolytes
Figure 2 and Table 2 shows the ionic conductivity for PVdF-HFP-[EMI-BTI] based SPEs samples
(SPE10–SPE40). Sample SPE30 shows the highest ionic conductivity at 3.92 µS¨cm´1 at room
temperature. This sample was then chosen to be the optimum conducting composition (OCC) sample
for this system and was then added with GO for further investigation of the effects of the addition of
GO. The increase of the ionic conductivity up to 30 wt % of the EMI-BTI added is due to the increasing
number of mobile ions [21] and also the plasticizing effect from the EMI-BTI ionic liquid [22,23].
However, there is a decrease in ionic conductivity that could be observed when 40 wt % of EMI-BTI
added into the system. This is due to the agglomeration of the excess ions from the EMI-BTI which
causes the mobile ions to form neutral pairs, and this could lead to a decrease in ionic mobility of the
SPEs [24].
Table 2. Ionic conductivity and activation energy values of the PVdF-HFP-[EMI-BTI]-based SPE
samples at room temperature.
Electrolytes
Ionic Conductivity (µS cm´1 )
Activation Energy, Ea (eV)
SPE10
SPE20
SPE30
SPE40
GO2
GO4
GO6
GO8
2.09
3.11
3.92
1.31
5.03
5.88
7.40
12.25
0.147
0.120
0.104
0.171
0.095
0.089
0.079
0.076
Materials 2016, 9, 450
Materials 2016, 9, 450
of 15
15
55 of
5 of 15
Figure 2. Ionic conductivity for the SPEs with different ratio of EMI-BTI and P(VdF-HFP) at room
Figure
conductivity
forfor
thethe
SPEs
withwith
different
ratioratio
of EMI-BTI
and P(VdF-HFP)
at room
Figure 2.
2. Ionic
Ionic
conductivity
SPEs
different
of EMI-BTI
and P(VdF-HFP)
at
temperature.
temperature.
room temperature.
The value of ionic conductivity for SPE samples with different weight percentages of GO is
The
value of
of ionicconductivity
conductivity
SPE
samples
different
weight
percentages
of GO is
Theinvalue
forfor
SPE
samples
withwith
different
weight
percentages
of GO
plotted
Figure ionic
3 and shown in Table
2. The
highest
value
of conductivity
observed
wasisatplotted
12.25
plotted
in
Figure
3
and
shown
in
Table
2.
The
highest
value
of
conductivity
observed
was
at
´1 at
−1 at3 8and
in Figure
in added.
Table 2. A
The
highest value
of conductivity
observed was at
12.25 µS¨upon
cm12.25
µS∙cm
wt shown
% of GO
significant
increase
in ionic conductivity
observed
the
−1 at 8 wt % of GO added. A significant increase in ionic conductivity was observed upon the
µS∙cm
8 wt % ofofGO
A significant
increase in
ionic
conductivity
observed upon
the addition
addition of
addition
theadded.
graphene
oxide nano-filler.
The
increase
of ionic was
conductivity,
after the
addition
of the
graphene
oxideThe
nano-filler.
The
increase
of ionic conductivity,
after
the addition
of
the graphene
oxide
nano-filler.
increase
of
ionic
conductivity,
addition
of graphene
oxide,
graphene
oxide,
is due
to the nature
of graphene
oxide
that has a after
lot ofthe
oxygen-containing
functional
graphene
oxide,
is
due
to
the
nature
of
graphene
oxide
that
has
a
lot
of
oxygen-containing
functional
is due tosuch
the as
nature
of graphene
oxide and
that hydroxyl
has a lot of
oxygen-containing
functional
groupsmobility
such as
groups
carboxylic
acid, epoxy
[25]
which can improve
charge carrier
groups
such
as
carboxylic
acid,
epoxy
and
hydroxyl
[25]
which
can
improve
charge
carrier
mobility
carboxylic
acid,
epoxy
and hydroxyl
whichby
can
improve
charge
carrierofmobility
andgroups,
promoteitionic
and
promote
ionic
conductivity
[26].[25]
Besides,
having
a huge
number
functional
also
and
promote [26].
ionicBesides,
conductivity
[26]. Besides,
by having
a huge number
of functional
groups,
it also
conductivity
by having
huge efficiently
number
ofin
functional
groups,
also helps
be
able
helps
GO to be able
to dissolve
muchamore
various types
of it
solvents,
andGO
in to
this
case,
helps
GO tomuch
be able
to dissolve
much
more efficiently
in various
types
of case,
solvents,
andcompared
in this case,
to
dissolve
more
efficiently
in
various
types
of
solvents,
and
in
this
acetone
to
acetone compared to other nanofillers such as the conventional silica (SiO2). This causes GO to be
acetone
compared
to
other
nanofillers
such
as
the
conventional
silica
(SiO
2). This causes GO to be
othertonanofillers
such asdistribute
the conventional
silica
(SiO
causesthe
GO
to be able
to homogeneously
able
homogeneously
in the SPEs,
and
could
facilitate
mobility
of mobile
charge ions
2 ). This
able
to homogeneously
distribute
in the SPEs,
and could
the mobility
of mobile
ions
distribute
in the
SPEs,inand
could
facilitate
the
mobility
offacilitate
mobile
charge
ions and
leads it
tocharge
increase
in
and
leads to
increase
ionic
conductivity
[27].
GO could
also act
as nanofiller
where
can
help to
and
leads
to
increase
in
ionic
conductivity
[27].
GO
could
also
act
as
nanofiller
where
it
can
help
to
ionic conductivity
[27]. GO could
also act
as nanofiller
where
it can
help
to promote
conducting
promote
more conducting
pathways
so that
the mobile
ions
move
easily
in the more
polymer
matrix.
promote
more
conducting
pathways
so
thatinthe
mobile
ions
move
the polymer
matrix.
pathways
that
the
mobile
ionsofmove
easily
theGO
polymer
matrix.
Aseasily
seenbased
ininFigure
the studies
of
As
seen inso
Figure
3, the
studies
the addition
of
into the
P(VdF-HFP)
SPE3,were
halted at
As
seen in Figure
3,into
thethe
studies
of the addition
of GO
into the P(VdF-HFP)
based
SPE were
halted at
GOadded.
P(VdF-HFP)
based studies
SPE
were
at 8for
wtthe
% of
the
GOofadded.
Supposedly,
8the
wtaddition
% of theof
GO
Supposedly,
further
arehalted
needed
addition
more GO
into the
8further
wt % of the GO
Supposedly,
furthermore
studies
are needed
for theHowever,
addition in
of this
more
GO into
the
areadded.
needed
the addition
system.
work,
film
system.studies
However,
in this for
work,
the filmoffor
10 GO
wt into
% ofthe
GO
doped SPE sample
was
toothe
brittle
system.
However,
in
this
work,
the
film
for
10
wt
%
of
GO
doped
SPE
sample
was
too
brittle
for 10 wt %further
of GOcharacterization
doped SPE sample
was too brittle rendering further characterization impossible.
rendering
impossible.
rendering further characterization impossible.
Figure 3. Ionic
Ionic conductivity
conductivity of
of PVdF-HFP-[EMI-BTI]-based
PVdF-HFP-[EMI-BTI]-based SPE
SPE samples
samples added
added with
with different
different weight
weight
Figure 3. Ionic conductivity of PVdF-HFP-[EMI-BTI]-based SPE samples added with different weight
percentages of GO at room temperature.
percentages of GO at room temperature.
Materials 2016, 9, 450
6 of 15
Materials 2016, 9, 450
6 of 15
3.2. Ion Conducting Mechanism
3.2. Ion Conducting Mechanism
Figure 4 shows the temperature dependence ionic conductivity for sample SPE10, SPE30, GO2
Figure
4 shows
the temperature
sample
SPE30, GO2
and GO8.
The
temperature
dependencedependence
studies wereionic
doneconductivity
in the range for
of 303
K to SPE10,
333 K. Temperature
and GO8. The
dependence
studies were
done
the range
of 303
K to 333 K.mechanism
Temperature
dependence
oftemperature
ionic conductivity
was analyzed
in order
toin
study
the ionic
conduction
of
dependence
of
ionic
conductivity
was
analyzed
in
order
to
study
the
ionic
conduction
mechanism
of
the SPE. It can be seen from the figure that the results presented regression lines close to unity (R ~ 1)
theall
SPE.
It can
be seenThe
from
the figure
that the
results
regression
to unity
(R ~ 1)
for
of the
samples.
linearity
of these
lines
showspresented
that the SPE
sampleslines
wereclose
obeying
Arrhenius
for
all
of
the
samples.
The
linearity
of
these
lines
shows
that
the
SPE
samples
were
obeying
Arrhenius
behaviour which can also be expressed as:
behaviour which can also be expressed as:
σ “ σ0 exp p´Ea {kTq
(2)
σ = σ0 exp (−Ea/kT)
(2)
By
By obeying
obeying the
the Arrhenius
Arrhenius behaviour,
behaviour, the
the ion
ion conduction
conduction mechanism
mechanism of all
all of
of the
the SPE
SPE samples
samples
was concluded
concludedto
tobe
be the
the hopping
hopping of
of the
the ions
ions from
from one
one site
site to another neighbouring
neighbouring site
site in
in the polymer
polymer
was
matrix. From
also
bebe
observed
that
there
waswas
an increase
in ionic
conductivity
with
matrix.
Fromthe
thefigures,
figures,ititcan
can
also
observed
that
there
an increase
in ionic
conductivity
the increase
of temperature.
ThisThis
is due
to to
thethe
higher
the
with
the increase
of temperature.
is due
highersegmental
segmentalmotion
motionofofpolymer
polymer chain
chain in the
amorphous region
region upon
upon addition
addition of
of heat.
heat. On
On the
the other
other hand,
hand, the
the activation
activation energy
energy (E
(Eaa) of
of the
the SPEs
SPEs
amorphous
was obtained
obtained from
from Figure
Figure 44 and
and was
was tabulated
tabulated in
in Table
Table 2.2. It
It can
can be
be seen
seen that
that sample
sample GO8
GO8 has
has the
the
was
lowest activation
activationenergy
energy
and
in agreement
with
the results
ionic conductivity
where
lowest
and
thisthis
is inisagreement
with the
results
in ionic in
conductivity
where normally
normally
sample
with the
ionic conductivity
have
the
activation
energy
the
samplethe
with
the higher
ionichigher
conductivity
would havewould
lowered
thelowered
activation
energy
[28]. The
Ea
[28]. The
Ea values
weretoalso
found to be to
comparable
to other
articles [29,30].
values
were
also found
be comparable
other articles
[29,30].
Figure
4. Arrhenius
plots
conductivity
of the
PVdF-HFP-[EMI-BTI]-based
SPE
samples.
Figure
4. Arrhenius
plots
forfor
thethe
conductivity
of the
PVdF-HFP-[EMI-BTI]-based
SPE
samples.
3.3. FTIR Measurements
Measurements of
of Solid
Solid Polymer
Polymer Electrolytes
Electrolytes
3.3.
In order
order to
to understand
understand the
the interaction
interaction between
between the
the polymer,
polymer, ionic
ionic liquid
liquid and
and GO,
GO, the
the IR
IR spectra
spectra
In
of pure
pure materials,
materials, SPE
SPE samples
samples with
with and
and without
without GO
GO added
added were
were presented
presented in
in Figure
Figure 5.
5. Some
Some of
of the
the
of
important band
band assignments
assignments has already been
been reported
reported for
for pure
pure P(VdF-HFP)
P(VdF-HFP) and
and EMI-BTI
EMI-BTI and
and the
the
important
band assignments
assignmentsare
arelisted
listedininTable
Table3 3[31,32].
[31,32].The
The
similar
FTIR
spectrum
been
reported
band
similar
FTIR
spectrum
of of
GOGO
hashas
been
reported
by
by Cao
et where
al., where
spectrum
shows
the presence
a different
oxygen
functional
group
in
Cao
et al.,
the the
GO GO
spectrum
shows
the presence
of a of
different
oxygen
functional
group
in the
´1 and
´1 were
the GO.
The peaks
3383
cm−1
andcm1645
cm−1assigned
were assigned
to O-H stretching
vibration
and
GO.
The peaks
at 3383atcm
1645
to O-H stretching
vibration and
stretching
stretching from
vibrations
from
C=O or
COOH, respectively
[33].
vibrations
C=O or
COOH,
respectively
[33].
−1, 1182
−1, and 1404
Initially, the addition
into
SPE
shifted
the peaks
at 796atcm
Initially,
additionofofEMI-BTI
EMI-BTI
into
SPE
shifted
the peaks
796
cm´1cm
, 1182
cm´1 , cm
and−1
1 of pure P(VdF-HFP)
1 , 1169
´−1
1 , respectively,
´1 ,SPE30.
−1,´and
of pure
to 791 cm−1, to
1169
1400cm
cm
The peak
1202
1404
cm´P(VdF-HFP)
791cm
cm
and 1400 cmin
respectively,
in at
SPE30.
´
1
−1
cm peak
of P(VdF-HFP)
nowhere to be
found
in SPE30.
This phenomenon
indicates
that there
were
The
at 1202 cm was
of P(VdF-HFP)
was
nowhere
to be found
in SPE30. This
phenomenon
indicates
interactions
between
the mobile
ions the
from
the EMI-BTI
ionic
and ionic
the polymer
matrix.
Fluorine
that
there were
interactions
between
mobile
ions from
theliquid
EMI-BTI
liquid and
the polymer
+ cation would
inside the
P(VdF-HFP)
is an
electron donating
atom donating
and this is
where
matrix.
Fluorine
inside the
P(VdF-HFP)
is an electron
atom
andthe
thisEMI
is where
the EMI+ form
cationa
dative form
bondawith
thebond
fluorine
form
P(VdF-HFP)-ionic
liquid complexes
On the [34].
otherOn
hand,
would
dative
withto
the
fluorine
to form P(VdF-HFP)-ionic
liquid[34].
complexes
the
upon the addition of GO, the peaks which were initially found in the spectrum of GO were nowhere
to be found in the spectra of SPE with GO (GO6 and GO8). Due to the disappearance of these peaks,
Materials 2016, 9, 450
7 of 15
Materials
2016, upon
9, 450 the addition of GO, the peaks which were initially found in the spectrum of GO7were
of 15
other hand,
nowhere to be found in the spectra of SPE with GO (GO6 and GO8). Due to the disappearance of these
it
was itbelieved
that the
groups
of the
GOGO
also
formed
a acomplexation
peaks,
was believed
thatfunctional
the functional
groups
of the
also
formed
complexationwith
withthe
the solid
solid
polymer
electrolytes.
polymer electrolytes.
Figure
GO6 and
and G8.
G8.
Figure 5.
5. FTIR
FTIR spectra
spectra of
of P(VdF-HFP),
P(VdF-HFP), EMI-BTI,
EMI-BTI, GO,
GO, sample
sample SPE30,
SPE30, GO6
Table
3. Possible
assignments
of some
peaks inpeaks
FTIR spectra
pure PVdF-HFP
EMI-BTI.
Table
3. Possible
assignments
of significant
some significant
in FTIRof spectra
of pure and
PVdF-HFP
and
EMI-BTI.
Samples
Wavenumber (cm−1)
Possible Assignments
Pure PVdF-HFP
762, 854
α-phase
Possible Assignments
796
CF3 stretching
Pure PVdF-HFP
762, 854
α-phase
838, 875
Amorphous
region
796
CF3 stretching
838, 875
Amorphous
region
1062
C-C skeletal
vibration
C-C skeletal vibration
11821062
Symmetrical
stretching of -CF3
1182
Symmetrical stretching of -CF3
12021202
Asymmetrical
stretching
of-CF
-CF
Asymmetrical
stretching of
2 - 21404
-C-F-strectching
1404
-C-F-strectching
Pure EMI-BTI
854
In-plane bending of imidazole ring
Pure EMI-BTI
854 949
In-plane bending
of imidazole
In-plane bending
of CNC ring
1352
Symmetrical
bending
in-plane
949
In-plane bending of CNCof CH
1386
In-plane asymmetrical stretching of imidazole ring
13521457
Symmetrical
bending in-plane of CH
Symmetrical in-play bending of imidazole ring
13861576
In-plane asymmetrical
stretching
of imidazole ring
HCN
deformation
2980
CH
stretching
in
methyl
group
1457
Symmetrical in-play bending of imidazole
ring
3120, 3157
=C-H stretching
15763587
HCNCH
deformation
stretching
2980
CH stretching in methyl group
=C-H stretching
3.4. X-ray Diffraction Studies3120, 3157
3587
CH stretching
Figure 6 shows the XRD pattern for pure PVdF-HFP, pure EMI-BTI, pure GO, SPE30 and the
GO-doped
P(VdF-HFP)-(EMI-BTI)-based
SPEs samples. As seen in the XRD pattern of the pure
3.4.
X-ray Diffraction
Studies
PVdF-HFP, the high intensity peak at around 2θ = 17˝ and 22.4˝ shows the highly crystalline nature of
Figure
6 shows However,
the XRD pattern
foraddition
pure PVdF-HFP,
EMI-BTI,
GO, the
SPE30
and the
the pure
PVdF-HFP.
upon the
of the ionicpure
liquid
as seen pure
in SPE30,
intensity
of
GO-doped
P(VdF-HFP)-(EMI-BTI)-based
SPEs
samples.
As
seen
in
the
XRD
pattern
of
the
pure
˝
˝
the peak at 2θ = 22.4 was found to be decreased and the peak at 2θ = 17 was found to be disappeared
PVdF-HFP,
thepattern.
high intensity peak at around 2θ = 17° and 22.4° shows the highly crystalline nature
from the XRD
of the pure PVdF-HFP. However, upon the addition of the ionic liquid as seen in SPE30, the intensity
of the peak at 2θ = 22.4° was found to be decreased and the peak at 2θ = 17° was found to be
disappeared from the XRD pattern.
This shows that the addition of the ionic liquid decreased the crystallinity of the backbone of the
PVdF-HFP and increased the amorphous region of the SPE samples [35]. This is in agreement with
Samples
Wavenumber (cm´1 )
disappearance of peaks show that there is some complexation occurring in the polymer matrix of the
SPE. The addition of GO was also found to decrease the intensity of the peak at 2θ = 23° as observed
in Figure 6. The intensity of the peak further decreased as more GO was added into the SPE. This
proves that the addition of GO increases the amorphous region of the SPE samples, which could assist
in improving the mobility of the mobile charge ions in the SPEs and leads to an increase in the ionic
Materials 2016, 9, 450
8 of 15
conductivity as observed in the previous studies [36].
Figure
6. XRD
different weight
weight
Figure 6.
XRD pattern
pattern for
for pure
pure P(VdF-HFP),
P(VdF-HFP), pure
pure EMI-BTI,
EMI-BTI, pure
pure GO,
GO, SPE30
SPE30 and
and different
percentage
of GO-doped
GO-doped PVdF-HFP-[EMI-BTI]-based
PVdF-HFP-[EMI-BTI]-based SPEs
SPEs samples.
samples.
percentage of
3.5. Thermogravimetric Analysis (TGA)
This shows that the addition of the ionic liquid decreased the crystallinity of the backbone of the
TGA thermograms
P(VdF-HFP),
pure [35].
GO, This
SPE30
and
the different
PVdF-HFP
and increasedfor
thepure
amorphous
regionpure
of theEMI-BTI,
SPE samples
is in
agreement
with
weight
SPEs arestudies.
shownFurthermore,
in Figure 7. the
Based
on the
the SPEs
what
haspercentage
been foundofinGO-doped
ionic conductivity
changes
in figure,
the intensity
and are
the
thermally
stable
up
to
around
440
°C
and
this
thermal
durability
is
desirable
for
most
electrochemical
disappearance of peaks show that there is some complexation occurring in the polymer matrix of the
device
applications.
Table
4, the
initial addition
of ionic
wasatfound
to˝decrease
the
SPE. The
addition ofAs
GOseen
wasin
also
found
to decrease
the intensity
ofliquid
the peak
2θ = 23
as observed
decomposition
of the
fromdecreased
501.28 °Casto
448.96
may
toThis
the
in Figure 6. Thetemperature
intensity of the
peakSPE
further
more
GO °C.
wasThis
added
intobethedue
SPE.
complexation
ionic liquid
with
the P(VdF-HFP)
which
has been
demonstrated
the XRD
studies
proves
that theofaddition
of GO
increases
the amorphous
region
of the
SPE samples,in
which
could
assist
that
decrease the
of the
SPE charge
and resulting
lower
stability.
in improving
the crystallinity
mobility of the
mobile
ions inin
the
SPEsthermal
and leads
to an increase in the ionic
As seen as
in observed
the figure,inpure
GO was studies
found to
be decomposing at a temperature around 194 °C
conductivity
the previous
[36].
because of the decomposition of the oxygen functional group and totally decomposed at 320 °C.
3.5. Thermogravimetric
Analysis
(TGA)and decomposes at high temperatures to form finely dispersed
Graphene
oxide normally
exfoliates
amorphous
carbon [22]. However,
as seen in thepure
figure
in the GO-doped
samples,
there
is no
TGA thermograms
for pure P(VdF-HFP),
EMI-BTI,
pure GO, SPE
SPE30
and the
different
trace of percentage
GO decomposing
at the temperature
aroundin194–320°C.
phenomenon
shows
there
weight
of GO-doped
SPEs are shown
Figure 7. This
Based
on the figure,
the that
SPEs
are
˝
are
complexations
of
the
GO
and
the
SPE
occurring
in
the
polymer
matrix
and
this
is
in
agreement
thermally stable up to around 440 C and this thermal durability is desirable for most electrochemical
with
XRD studies.
It is in
also
found
the addition
additionofofionic
GOliquid
only was
slightly
decreases
the
devicethe
applications.
As seen
Table
4, thethat
initial
found
to decrease
˝
˝
decomposition
temperature
of
the
SPE.
As
seen
in
the
table,
the
decomposition
temperature
of
SPE30
the decomposition temperature of the SPE from 501.28 C to 448.96 C. This may be due to the
that
was at 448.96
°C liquid
decreases
448.76
°C, 444.53
°C, has
438.95
°Cdemonstrated
and 437.84 °Cinfor
GO8,
complexation
of ionic
withtothe
P(VdF-HFP)
which
been
theGO2
XRDtostudies
respectively.
shows that of
thethe
addition
ofresulting
GO only in
slightly
the thermal stability of the
that decrease This
the crystallinity
SPE and
lower decreases
thermal stability.
SPE by
a
negligible
value.
As seen in the figure, pure GO was found to be decomposing at a temperature around 194 ˝ C
because of the decomposition of the oxygen functional group and totally decomposed at 320 ˝ C.
Graphene oxide normally exfoliates and decomposes at high temperatures to form finely dispersed
amorphous carbon [22]. However, as seen in the figure in the GO-doped SPE samples, there is no trace
of GO decomposing at the temperature around 194–320 ˝ C. This phenomenon shows that there are
complexations of the GO and the SPE occurring in the polymer matrix and this is in agreement with
the XRD studies. It is also found that the addition of GO only slightly decreases the decomposition
temperature of the SPE. As seen in the table, the decomposition temperature of SPE30 that was at
448.96 ˝ C decreases to 448.76 ˝ C, 444.53 ˝ C, 438.95 ˝ C and 437.84 ˝ C for GO2 to GO8, respectively.
This shows that the addition of GO only slightly decreases the thermal stability of the SPE by a
negligible value.
Materials 2016, 9, 450
Materials 2016, 9, 450
9 of 15
9 of 15
Figure
7. TGA
TGAthermograms
thermogramsforfor
pure
P(VdF-HFP),
EMI-BTI,
GO, SPE30
and different
Figure 7.
pure
P(VdF-HFP),
purepure
EMI-BTI,
pure pure
GO, SPE30
and different
weight
weight
percentages
of
GO-doped
PVdF-HFP-[EMI-BTI]-based
SPE
samples.
percentages of GO-doped PVdF-HFP-[EMI-BTI]-based SPE samples.
Table
PVdF-HFP-[EMI-BTI]-based SPE
SPE samples.
samples.
Table 4.
4. Decomposition
Decomposition temperature
temperature for
for PVdF-HFP-[EMI-BTI]-based
Samples
Pure PVdF-HFP
Pure PVdF-HFP
Pure EMI-BTI
Pure EMI-BTI
Pure GO
Pure GO
SPE30
SPE30
GO2 GO2
GO4 GO4
GO6
GO6
GO8
GO8
Samples
Decomposition Temperature, TD (°C)
Decomposition Temperature, T D (˝ C)
501.28
500.15 501.28
500.15
194.08 194.08
448.96 448.96
448.78 448.78
444.53 444.53
438.95
438.95
437.84
437.84
3.6. Electrochemical Impedance (EIS) Performance of the EDLC
3.6. Electrochemical Impedance (EIS) Performance of the EDLC
The electrochemical performance of AC/GO-doped SPE/AC supercapacitors were evaluated
The electrochemical performance of AC/GO-doped SPE/AC supercapacitors were evaluated
using CV analysis. Cyclic voltammograms were performed for GO2, GO4, GO6 and GO8 with a
using CV analysis. Cyclic voltammograms were performed for GO2, GO4, GO6 and GO8 with a
potential window from ´1 to 1 V at 5, 10, 20, 30, 40, and 50 mV/s scan rates.
potential window from −1 to 1 V at 5, 10, 20, 30, 40, and 50 mV/s scan rates.
Using electrode mass, the currents measured were normalized. The specific capacitance C can
Using electrode mass, the currents measured were normalized. The specific capacitance Csp
sp can
be obtained using the equation below:
be obtained using the equation below:
It
Csp “
(3)
MvV
(3)
where It is the area of the CV curve, m is the mass of active material, v is the scan rates and V is a
potential
window.
shows
them
CV
of GO2,
GO4,
GO6 and
GO8
different
scanVrates,
where
It is
the areaFigure
of the8CV
curve,
is curves
the mass
of active
material,
v is
theatscan
rates and
is a
respectively.
CV curves
havethe
rectangular
exhibited
excellent
electrochemical
stability,
potential
window.
Figurethat
8 shows
CV curvesshapes
of GO2,
GO4, GO6
and GO8
at different scan
rates,
especially at the
rate. rectangular
Figure 9 shows
a rectangular
CV, which
is nearly mirror
image
respectively.
CV higher
curves scan
that have
shapes
exhibited excellent
electrochemical
stability,
symmetry at
of the
the higher
currentscan
responses
to zero
line. This
confirms the
layer
capacitive
especially
rate. Figure
9 shows
a rectangular
CV,electric
which double
is nearly
mirror
image
behavior [37].
In addition,
reversible
humps
also
observed
(atelectric
a scan rate
of 3 mV/s),
revealing
symmetry
of the
current small
responses
to zero
line.are
This
confirms
the
double
layer capacitive
that redox[37].
reactions
are alsosmall
contributing
to the
specific
These
behavior
In addition,
reversible
humps
arecapacitance.
also observed
(at small
a scanhumps
rate ofare
3 arising
mV/s),
+
from the fast
of Na are
in the
of activated
electrodes.
In fact,
electronegative
revealing
thatswitching
redox reactions
alsopores
contributing
to thecarbon
specific
capacitance.
These
small humps
+ in the
functional
on GO
servedofasNa
channels
forpores
rapidof
switching
of carbon
Na+ by interacting
the
are
arisinggroups
from the
fastsheets
switching
activated
electrodes. with
In fact,
+
ions.
However,
in
the
case
of
cells
using
GPE
without
GO,
redox
peaks
were
not
prominent,
confirming
electronegative functional groups on GO sheets served as channels for rapid switching of Na by
that in the absence
GO,
intercalation
Na+ofdid
not
prevail.
werepeaks
found
by G.P.
interacting
with the of
ions.
However,
in theofcase
cells
using
GPE Similar
withoutresults
GO, redox
were
not
+ did
Pandey et al.
[38,39]. The
calculated
specific
capacitance
of GO2,
GO4,
GO6
and GO8Similar
were 20
F/g,
prominent,
confirming
that
in the absence
of GO,
intercalation
of Na
not prevail.
results
24 F/g,
29.6 by
F/gG.P.
andPandey
24 F/g et
at al.
a scan
rateThe
of 3calculated
mV/s respectively.
were
found
[38,39].
specific capacitance of GO2, GO4, GO6 and
GO8 were 20 F/g, 24 F/g, 29.6 F/g and 24 F/g at a scan rate of 3 mV/s respectively.
Materials 2016, 9, 450
Materials 2016, 9, 450
Materials 2016, 9, 450
10 of 15
10 of 15
10 of 15
Figure
specificcapacitance
capacitanceatat
atdifferent
different
scan
rates
for
EDLCs
fabricated
with
sample
Figure
8.8.Variation
capacitance
different
scan
rates
EDLCs
fabricated
with
sample
Figure8.
Variation in
in specific
specific
scan
rates
forfor
EDLCs
fabricated
with
sample
(a)
GO2;
(b)
GO4;
(c)
GO6;
(d)
GO8.
(a)
GO2;
(b)
GO4;
(c)
GO6;
(d)
GO8.
(a) GO2; (b) GO4; (c) GO6;
GO8.
Figure 9. CV curve for EDLC fabricated with SPE with (GO6) and without (SPE30) GO at scan rate of
Figure
9. CV
GO
at at
scan
rate
of
3 mV/s.
Figure
9.
CV curve
curve for
for EDLC
EDLCfabricated
fabricatedwith
withSPE
SPEwith
with(GO6)
(GO6)and
andwithout
without(SPE30)
(SPE30)
GO
scan
rate
3
mV/s.
of 3 mV/s.
The cyclic discharge can further evaluate the electrochemical capacitance of materials. Figure 10
Thethe
cyclic
dischargedischarge
can further
evaluate
theassembled
electrochemical
capacitance
Figure 10
shows
galvanostatic
curves
for the
supercapacitor
cells of
at materials.
current densities
The cyclic discharge can further evaluate the electrochemical capacitance of materials. Figure 10
of 50, 100,
200, 300 mA/g discharge
at room temperature
GO dopedsupercapacitor
SPEs. It is evident
from
the discharge
shows
the galvanostatic
curves forfor
thethe
assembled
cells
at current
densities
shows
the
galvanostatic
discharge
curves
for the assembled
supercapacitor
cells at current
densities of
that
assembled
exhibit
linear characteristics
represent
outstanding
electrochemical
ofcurves
50, 100,
200,
300 mA/gcells
at room
temperature
for the GOthat
doped
SPEs. It
is evident from
the discharge
50,
100, 200, 300
mA/g
at room temperature
for the GO doped SPEs. It is evident from the discharge
performance
from
the supercapacitor
[40]. characteristics
curves
that assembled
cells exhibit linear
that represent outstanding electrochemical
curves that assembled cells exhibit linear characteristics that represent outstanding electrochemical
performance from the supercapacitor [40].
performance from the supercapacitor [40].
Materials 2016, 9, 450
11 of 15
Materials 2016, 9, 450
11 of 15
Materials 2016, 9, 450
11 of 15
Figure
10. Galvanostatic
discharge
curves
for for
EDLCs
fabricated
with
sample
(a)(a)
GO2;
(b)(b)
GO4;
(c) GO6;
Figure
10. Galvanostatic
discharge
curves
EDLCs
fabricated
with
sample
GO2;
GO4;
Figure 10. Galvanostatic discharge curves for EDLCs fabricated with sample (a) GO2; (b) GO4;
(d) GO8.
(c) GO6; (d) GO8.
(c) GO6; (d) GO8.
For further explanation of the high electrochemical performance, an electrochemical impedance
For
explanation
of the
electrochemical performance,
an
impedance
For further
further
explanation
the high
high
electrochemical
performance,
an electrochemical
electrochemical
impedance
spectrum
(EIS)
was used toofexamine
GO-doped
SPE-based
EDLCs. The equivalent
series resistance
spectrum
(EIS)
was
used
to
examine
GO-doped
SPE-based
EDLCs.
The
equivalent
series
spectrum
(EIS) was
used tofrom
examine
GO-dopedofSPE-based
series resistance
resistance
(ESR) could
be obtained
the intersection
the curves EDLCs.
with the The
axis equivalent
of real impedance.
The
(ESR)
could
be
obtained
from
the
intersection
of
the
curves
with
the
axis
of
real
impedance.
(ESR)
could be
obtained
the intersection
of the curves
the conductances
axis of real of
impedance.
The
difference
in the
ESRs offrom
supercapacitor
cells is attributed
to the with
different
electrode The
difference
in
of
is
the
different
conductances
of
materials
[41].ESRs
It is evident
from Figurecells
11 that
the ESR ofto
GO6
was found
to be minimal
as
difference
in the
the
ESRs
of supercapacitor
supercapacitor
cells
is attributed
attributed
tothe
the
different
conductances
of electrode
electrode
materials
[41].
is evident
evident
from
Figure
11 that
that the
the
ESR
of
the GO6
was
found
to
compared
and GO8.
This
also confirms
the ESR
high of
conductive
electrochemical
materials
[41].toItItGO4
is
from
Figure
11
the
GO6 and
wasgood
found
to be
be minimal
minimal as
as
performance
of
the
EDLC
fabricated
with
GO6.
compared
to
GO4
and
GO8.
This
also
confirms
the
high
conductive
and
good
electrochemical
compared to GO4 and GO8. This also confirms the high conductive and good electrochemical
performance
performance of
of the
the EDLC
EDLC fabricated
fabricated with
with GO6.
GO6.
Figure 11. Nyquist Plots for EDLCs fabricated with PVdF-HFP-[EMI-BTI]-based SPE samples with
different weight percentages of GO.
Figure 11. Nyquist Plots for EDLCs fabricated with PVdF-HFP-[EMI-BTI]-based SPE samples with
Figure 11. Nyquist Plots for EDLCs fabricated with PVdF-HFP-[EMI-BTI]-based SPE samples with
different weight
weight percentages
percentages of
of GO.
GO.
different
Materials 2016, 9, 450
12 of 15
Materials 2016, 9, 450
12 of 15
Long-term
Long-term cycling
cycling stability
stability test
test was
was aa key
key factor
factor to
to evaluate
evaluate the
the SPE
SPE performance
performance for
for practical
practical
applications.
The
long-term
cycling
performance
over
3000
cycles
of
GO6
was
analyzed
by
applications. The long-term cycling performance over 3000 cycles of GO6 was analyzed by repeating
repeating
the
test at
at aa current
current density
density of
of 200
200 mA/g
mA/gand
andthe
the results
results are
are shown
shown in
in Figure
Figure 12.
12.
the charge/discharge
charge/discharge test
Initially,
the
specific
capacitance
increased
at
the
start
of
charge–discharge
cycling.
This
increment
Initially, the specific capacitance increased at the start of charge–discharge cycling. This increment in
in
specific
to to
thethe
activation
of carbon
electrodes
on charge–discharge
cycling.cycling.
When
specific capacitance
capacitancewas
wasdue
due
activation
of carbon
electrodes
on charge–discharge
all
electrode
sites are
activated,
specific
capacitance
reached
its peak
However,
with
When
all electrode
sites
are activated,
specific
capacitance
reached
its until
peak 800
untilcycles.
800 cycles.
However,
further
charge–discharge
cycling,
specific
capacitance
decreased
slowly.
After
charge–discharge
cycling
with further charge–discharge cycling, specific capacitance decreased slowly. After charge–discharge
of
3000 cycles,
capacitive
retention retention
dropped by
only 6%.
are comparable
to the already
cycling
of 3000the
cycles,
the capacitive
dropped
byOur
onlyresults
6%. Our
results are comparable
to
reported
results
summarized
in
Table
5.
This
confirmed
the
excellent
stability
of
GO-based
SPEs
for
the already reported results summarized in Table 5. This confirmed the excellent stability of GOenergy
storage
applications.
these
results indicate
that GO6
is a promising
for
based SPEs
for energy
storageTherefore,
applications.
Therefore,
these results
indicate
that GO6 iscandidate
a promising
high-performance
energy storageenergy
systems.
candidate for high-performance
storage systems.
Figure 12.
12. The
function of
of cycle
cycle number
number from
from 5th
5th to
to 3000th
3000th
Figure
The variation
variation of
of specific
specific capacitance
capacitance as
as aa function
for GO6.
Table 5.
5. Summary
Summary of
of reported
reported results
results similar
similar to
to this
this work.
work.
Table
Electrolyte
Electrolyte
Material
Material
Current
Electrodes
(A/g)
ElectrodesCurrent Density
Density
PVA–H2SO4
Activated carbon
PVA–H2SO4 Activated
Activated
HQ-H2SO4
carbon carbon
HQ-H2SO4 Activated
Activated
PVDF-HFP+GO
carbon carbon
PVDF-HFP+GO
Activated carbon
(A/g)
Nil
Nil Nil
0.2 Nil
0.2
Total
Cycles
Total Cycles
3000
3000
4000
4000
3000
3000
Capacitive
Capacitive
Retention
(%)
Retention
(%)
90.77
90.7765
65 94
94
Reference
Reference
[42]
[42]
[43]
[43]work
This
This work
4. Conclusions
4. Conclusions
In this work, solid polymer electrolyte (SPE) films composed of semi-crystalline P(VdF-HFP)
In this as
work,
(SPE)
composed
semi-crystalline
P(VdF-HFP)
copolymer
the solid
host polymer
polymer electrolyte
and EMI-BTI
asfilms
the ionic
liquidofwere
synthesized,
and their
copolymer aswere
the evaluated
host polymer
and EMI-BTI
thecapacitors
ionic liquid
wereThe
synthesized,
their
performances
for electrical
double as
layer
(EDLC).
addition of and
graphene
performances
were
evaluated
for
electrical
double
layer
capacitors
(EDLC).
The
addition
of
graphene
oxide as filler does help in enhancing the ionic conductivity of the solid polymer electrolyte. The
oxide as fillerofdoes
help in enhancing
ionic
of theofsolid
polymer
electrolyte.
The
conductivity
the electrolyte
increasesthe
with
the conductivity
weight percentage
graphene
oxide.
The highest
conductivity of the electrolyte increases with the weight percentage of graphene oxide. The highest
conductivity obtained was 12.25 µS cm´1 for GO8. A temperature dependence study of the GO-doped
conductivity obtained was 12.25 µS cm−1 for GO8. A temperature dependence study of the GO-doped
SPE showed that the ionic conduction mechanism follows Arrhenius. These SPEs are thermally stable
SPE showed that the ionic conduction mechanism follows Arrhenius. These SPEs are thermally stable
up to 400 ˝ C with the addition of ionic liquid-formed complexation with the polymer, while GO
up to 400 °C with the addition of ionic liquid-formed complexation with the polymer, while GO
addition only decreases the decomposition temperature slightly. The electrochemical performance
addition only decreases the decomposition temperature slightly. The electrochemical performance
studies of GO-doped SPE supercapacitors showed that GO6 containing 6% of GO had a maximum
studies of GO-doped SPE supercapacitors showed that GO6 containing 6% of GO had a maximum
specific capacitance of 29.6 F/g at a scan rate of 3 mV s´1 compared to other SPEs due to high
specific capacitance of 29.6 F/g at a scan rate of 3 mV s−1 compared to other SPEs due to high
conductivity and effective ion transfer. Moreover, GO6 SPE-based EDLCs exhibited an excellent
Materials 2016, 9, 450
13 of 15
conductivity and effective ion transfer. Moreover, GO6 SPE-based EDLCs exhibited an excellent cyclic
retention of 94% even after 3000 cycles. These results suggest that GO-doped SPE can play a vital role
in energy storage applications.
Acknowledgments: This work was supported by the High Impact Research Grant (H-21001-F000046) and
Fundamental Research Grant Scheme (FP012-2015A) from the Ministry of Education, Malaysia. The authors
would like to express their appreciation for financial support from grant No. PG034-2013A from the Institute of
Research Management & Monitoring (IPPP) of the University of Malaya.
Author Contributions: H.M.N., Y.K.M., A.N., S.R. and K.R. conducted and supervised the project, including the
supports for characterizations. N.F.A.F. performed most of the experiments. All authors reviewed the manuscript.
Conflicts of Interest: The authors declare no conflict of interest.
References
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
Sudhakar, Y.N.; Selvakumar, M.; Bhat, D.K. Tubular array, dielectric, conductivity and electrochemical
properties of biodegradable gel polymer electrolyte. Mater. Sci. Eng. B 2014, 180, 12–19. [CrossRef]
Yang, X.; Zhang, F.; Zhang, L.; Zhang, T.; Huang, Y.; Chen, Y. A high-performance graphene oxide-doped
ion gel as gel polymer electrolyte for all-solid-state supercapacitor applications. Adv. Funct. Mater. 2013, 23,
3353–3360. [CrossRef]
Manuel Stephan, A.; Nahm, K.S. Review on composite polymer electrolytes for lithium batteries. Polymer
2016, 47, 5952–5964. [CrossRef]
Abbrent, S.; Plestil, J.; Hlavata, D.; Lindgren, J.; Tegenfeldt, J.; Wendsjö, Å. Crystallinity and morphology of
PVdF–HFP-based gel electrolytes. Polymer 2001, 42, 1407–1416. [CrossRef]
Stolarska, M.; Niedzicki, L.; Borkowska, R.; Zalewska, A.; Wieczorek, W. Structure, transport properties and
interfacial stability of PVdF/HFP electrolytes containing modified inorganic filler. Electrochim. Acta 2007, 53,
1512–1517. [CrossRef]
Manuel Stephan, A.; Nahm, K.S.; Anbu Kulandainathan, M.; Ravi, G.; Wilson, J. Electrochemical studies on
nanofiller incorporated poly(vinylidene fluoride-hexafluoropropylene) (PVdF-HFP) composite electrolytes
for lithium batteries. J. Appl. Electrochem. 2006, 36, 1091–1097. [CrossRef]
Lu, J.; Yan, F.; Texter, J. Advanced applications of ionic liquids in polymer science. Prog. Polym. Sci. 2009, 34,
431–448. [CrossRef]
Joshi, R.; Pasilis, S.P. The effect of tributylphosphate and tributyl phosphine oxide on hydrogen bonding
interactions between water and the 1-ethyl-3-methylimidazolium cation in 1-ethyl-3-methylimidazolium
bis(trifluoromethylsulfonyl)imide. J. Mol. Liq. 2015, 209, 381–386. [CrossRef]
Ravi, M.; Kiran Kumar, K.; Madhu Mohan, V.; Narasimha Rao, V.V.R. Effect of nano TiO2 filler on the
structural and electrical properties of PVP based polymer electrolyte films. Polym. Test. 2014, 33, 152–160.
[CrossRef]
Choi, Y.R.; Yoon, Y.-G.; Choi, K.S.; Kang, J.H.; Shim, Y.-S.; Kim, Y.H.; Chang, H.J.; Lee, J.-H.; Park, C.R.;
Kim, S.Y. Role of oxygen functional groups in graphene oxide for reversible room-temperature NO2 sensing.
Carbon 2016, 91, 178–187. [CrossRef]
Jayalakshmi, M.; Balasubramanian, K. Simple capacitors to supercapacitors-An overview. Int. J. Electrochem.
Sci. 2008, 3, 1196–1217.
Numan, A.; Duraisamy, N.; Saiha Omar, F.; Mahipal, Y.K.; Ramesh, K.; Ramesh, S. Enhanced electrochemical
performance of cobalt oxide nanocube intercalated reduced graphene oxide for supercapacitor application.
RSC Adv. 2016, 6, 34894–34902. [CrossRef]
Liu, H.; Zhang, L.; Guo, Y.; Cheng, C.; Yang, L.; Jiang, L.; Yu, G.; Hu, W.; Liu, Y.; Zhu, D. Reduction of
graphene oxide to highly conductive graphene by Lawesson’s reagent and its electrical applications. J. Mater.
Chem. C 2013, 1, 3104. [CrossRef]
Rajendran, R.; Shrestha, L.K.; Minami, K.; Subramanian, M.; Jayavel, R.; Ariga, K. Dimensionally integrated
nanoarchitectonics for a novel composite from 0D, 1D, and 2D nanomaterials: RGO/CNT/CeO 2 ternary
nanocomposites with electrochemical performance. J. Mater. Chem. A 2014, 2, 18480–18487. [CrossRef]
Rajendran, R.; Shrestha, L.K.; Kumar, R.M.; Jayavel, R.; Hill, J.P.; Ariga, K. Composite nanoarchitectonics for
ternary systems of reduced graphene oxide/carbon nanotubes/nickel oxide with enhanced electrochemical
capacitor performance. J. Inorg. Organomet. Polym. Mater. 2015, 25, 267–274. [CrossRef]
Materials 2016, 9, 450
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
26.
27.
28.
29.
30.
31.
32.
33.
34.
35.
36.
37.
14 of 15
Merlet, C.; Rotenberg, B.; Madden, P.A.; Taberna, P.-L.; Simon, P.; Gogotsi, Y.; Salanne, M. On the molecular
origin of supercapacitance in nanoporous carbon electrodes. Nat. Mater. 2012, 11, 306–310. [CrossRef]
[PubMed]
Liew, C.W.; Ramesh, S.; Arof, A.K. Characterization of ionic liquid added poly(vinyl alcohol)-based proton
conducting polymer electrolytes and electrochemical studies on the supercapacitors. Int. J. Hydrog. Energy
2015, 40, 852–862. [CrossRef]
Peik-See, T.; Pandikumar, A.; Ngee, L.H.; Ming, H.N.; Hua, C.C. Magnetically separable reduced graphene
oxide/iron oxide nanocomposite materials for environmental remediation. Catal. Sci. Technol. 2014, 4,
4396–4405. [CrossRef]
Nadiah, N.S.; Mahipal, Y.K.; Numan, A.; Ramesh, S.; Ramesh, K. Efficiency of supercapacitor using
EC/DMC-based liquid electrolytes with methyl methacrylate (MMA) monomer. Ionics 2015, 22, 1–8.
[CrossRef]
Qian, X.; Gu, N.; Cheng, Z.; Yang, X.; Wang, E.; Dong, S. Methods to study the ionic conductivity of polymeric
electrolytes using A.C. impedance spectroscopy. J. Solid State Electrochem. 2001, 6, 8–15. [CrossRef]
Zhang, R.; Chen, Y.; Montazami, R. Ionic liquid-doped gel polymer electrolyte for flexible lithium-ion
polymer batteries. Materials 2015, 8, 2735–2748. [CrossRef]
Shamsuri, A.A.; Daik, R. Applications of ionic liquids and their mixtures for preparation of advanced
polymer blends and composites: A short review. Rev. Adv. Mater. Sci. 2014, 40, 45–59.
Ramesh, S.; Liew, C.W.; Arof, A.K. Ion conducting corn starch biopolymer electrolytes doped with ionic liquid
1-butyl-3-methylimidazolium hexafluorophosphate. J. Non-Cryst. Solids 2011, 357, 3654–3660. [CrossRef]
Ye, Y.S.; Wang, H.; Bi, S.G.; Xue, Y.; Xue, Z.G.; Liao, Y.G.; Zhou, X.P.; Xie, X.L.; Mai, Y.W. Enhanced ion
transport in polymer–ionic liquid electrolytes containing ionic liquid-functionalized nanostructured carbon
materials. Carbon 2015, 86, 86–97. [CrossRef]
Dreyer, D.R.; Park, S.; Bielawski, C.W.; Ruoff, R.S. The chemistry of graphene oxide. Chem. Soc. Rev. 2010, 39,
228–240. [CrossRef] [PubMed]
Shim, J.; Kim, D.-G.; Kim, H.J.; Lee, J.H.; Baik, J.-H.; Lee, J.-C. Novel composite polymer electrolytes
containing poly(ethylene glycol)-grafted graphene oxide for all-solid-state lithium-ion battery applications.
J. Mater. Chem. A 2014, 2, 13873. [CrossRef]
Ye, Y.-S.; Rick, J.; Hwang, B.-J. Ionic liquid polymer electrolytes. J. Mater. Chem. A 2013, 1, 2719–2743.
[CrossRef]
Khan, M.S.; Shakoor, A. Ionic conductance, thermal and morphological behaviour of peo-graphene
oxide-salts composites. J. Chem. 2015. [CrossRef]
Ahmad, S.; Deepa, M.; Agnihotry, S.A. Effect of salts on the fumed silica-based composite polymer
electrolytes. Sol. Energy Mater. Sol. Cells 2008, 92, 184–189. [CrossRef]
Sivakumar, M.; Subadevi, R.; Rajendran, S.; Wu, H.-C.; Wu, N.-L. Compositional effect of PVdF–PEMA blend
gel polymer electrolytes for lithium polymer batteries. Eur. Polym. J. 2007, 43, 4466–4473. [CrossRef]
Ramesh, S.; Lu, S.-C.; Vankelecom, I. BMIMTf ionic liquid-assisted ionic dissociation of MgTf in
P(VdF-HFP)-based solid polymer electrolytes. J. Phys. Chem. Solids 2013, 74, 1380–1386. [CrossRef]
Sa’adun, N.N.; Subramaniam, R.; Kasi, R. Development and characterization of poly(1-vinylpyrrolidoneco-vinyl acetate) copolymer based polymer electrolytes. Sci. World J. 2014, 2014, 1–7. [CrossRef] [PubMed]
Cao, Y.C.; Xu, C.; Wu, X.; Wang, X.; Xing, L.; Scott, K. A poly (ethylene oxide)/graphene oxide electrolyte
membrane for low temperature polymer fuel cells. J. Power Sources 2011, 196, 8377–8382. [CrossRef]
Noor, M.M.; Buraidah, M.H.; Careem, M.A.; Majid, S.R.; Arof, A.K. An optimized poly(vinylidene
fluoride-hexafluoropropylene)-NaI gel polymer electrolyte and its application in natural dye sensitized solar
cells. Electrochim. Acta 2014, 121, 159–167. [CrossRef]
Noor, M.M.; Careem, M.A.; Majid, S.R.; Arof, A.K. Characterisation of plasticised PVDF–HFP polymer
electrolytes. Mater. Res. Innov. 2011, 15, 157–160. [CrossRef]
Yang, X.; Zhang, L.; Zhang, F.; Zhang, T.; Huang, Y.; Chen, Y. A high-performance all-solid-state
supercapacitor with graphene-doped carbon material electrodes and a graphene oxide-doped ion gel
electrolyte. Carbon 2014, 72, 381–386. [CrossRef]
Pandey, G.P.; Hashmi, S.A. Ionic liquid 1-ethyl-3-methylimidazolium tetracyanoborate-based gel polymer
electrolyte for electrochemical capacitors. J. Mater. Chem. A 2013, 1, 3372–3378. [CrossRef]
Materials 2016, 9, 450
38.
39.
40.
41.
42.
43.
15 of 15
Pandey, G.P.; Hashmi, S.A. Solid-state supercapacitors with ionic liquid based gel polymer electrolyte: Effect
of lithium salt addition. J. Power Sources 2013, 243, 211–218. [CrossRef]
Lim, C.-S.; Teoh, K.H.; Liew, C.-W.; Ramesh, S. Capacitive behaviour studies on electrical double layer
capacitor using poly (vinyl alcohol)-lithium perchlorate based polymer electrolyte incorporated with TiO2 .
Mater. Chem. Phys. 2014, 143, 661–667. [CrossRef]
Teoh, K.H.; Lim, C.-S.; Liew, C.-W.; Ramesh, S.; Ramesh, S. Electric double-layer capacitors with corn
starch-based biopolymer electrolytes incorporating silica as filler. Ionics 2015, 21, 2061–2068. [CrossRef]
Lu, J.; Zhang, Y.; Lu, Z.; Huang, X.; Wang, Z.; Zhu, X.; Wei, B. A preliminary study of the pseudo-capacitance
features of strontium doped lanthanum manganite. RSC Adv. 2015, 8, 5858–5862. [CrossRef]
Yu, H.; Wu, J.; Fan, L.; Lin, Y.; Xu, K.; Tang, Z.; Cheng, C.; Tang, S.; Lin, J.; Huang, M.; et al. A novel
redox-mediated gel polymer electrolyte for high-performance supercapacitor. J. Power Sources 2012, 198,
402–407. [CrossRef]
Roldn, S.; Blanco, C.; Granda, M.; Menndez, R.; Santamar, R. Towards a further generation of high-energy
carbon-based capacitors by using redox-active electrolytes. Angew. Chem. Int. Ed. 2011, 50, 1699–1701.
[CrossRef] [PubMed]
© 2016 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC-BY) license (http://creativecommons.org/licenses/by/4.0/).