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Characterization of ionic liquid addedpoly(vinyl alcohol)-based proton conductingpolymer electrolytes and electrochemicalstudies on the supercapacitors
Chiam-Wen Liew, S. Ramesh*, A.K. Arof
Centre for Ionics University of Malaya, Department of Physics, Faculty of Science, University of Malaya,
Lembah Pantai, 50603 Kuala Lumpur, Malaysia
a r t i c l e i n f o
Article history:
Received 25 March 2014
Received in revised form
10 September 2014
Accepted 29 September 2014
Available online 24 October 2014
Keywords:
Poly(vinyl alcohol)
Ionic liquid
Proton conductive
Supercapacitors
Carbon nanotubes
* Corresponding author. Tel.: þ60 3 7967 439E-mail addresses: liewchiamwen85@gma
http://dx.doi.org/10.1016/j.ijhydene.2014.09.10360-3199/Copyright © 2014, Hydrogen Energ
a b s t r a c t
The preparation of poly(vinyl alcohol) (PVA)/ammonium acetate (CH3COONH4)/1-butyl-3-
methylimidazolium bromide (BmImBr) proton conducting polymer electrolytes is done by
solution casting method. Upon inclusion of 60 wt.% of BmImBr, the maximum ionic con-
ductivity of (9.29 ± 0.01) mScm�1 is achieved at ambient temperature. Ionic liquid added
polymer electrolytes exhibit lower glass transition temperature (Tg), crystalline melting
temperature (Tm) and crystallization temperature (Tc) than ionic liquid-free polymer elec-
trolyte. The amorphous character of the most conducting polymer electrolyte has been
proven using differential scanning calorimetry (DSC). Addition of ionic liquid not only
extends the electrochemical potential window of the electrolyte, but also improves the
thermal stability of the polymer electrolyte. Activated carbon/carbon black/carbon nano-
tube electrode is prepared and used in electrochemical double layer capacitors (EDLCs)
fabrication. Based on the results, EDLC containing ionic liquid added polymer electrolyte
exhibits better electrochemical properties. This EDLC possesses higher specific capacitance
than that of supercapacitor comprising of ionic liquid free-based polymer electrolyte. The
specific capacitance of 21.89 Fg�1 is obtained from cyclic voltammetry (CV). This value is in
good agreement with EIS and galvanostatic chargeedischarge findings. The EDLC remains
stable upon 250 cycles of charging and discharging processes.
Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
reserved.
Introduction
Electrochemical double layer capacitor (EDLC) is an energy
storage-based electrochemical devices. ELDC generally com-
prises two electrodes and an ion conducting electrolyte.
Activated carbon (AC) is a predominant electrode material
1; fax: þ60 3 7967 4146.il.com (C.-W. Liew), rame60y Publications, LLC. Publ
used in EDLCs because of its attractive properties. Large spe-
cific surface area (1000e2500 m2 g�1), high porosity and low
cost are the advantages of AC [1,2]. However, high micropo-
rosity (pore dimension: <2 nm) of activated carbon could limit
the accessibility of charge carriers into the micropores of AC.
It is because the bigger ion size serves as a hurdle for diffusion
into the smaller pores [1,3]. Therefore, carbon nanotubes
shtsubra@gmail.com (S. Ramesh).
ished by Elsevier Ltd. All rights reserved.
i n t e r n a t i o n a l j o u rn a l o f h y d r o g e n en e r g y 4 0 ( 2 0 1 5 ) 8 5 2e8 6 2 853
(CNTs) which havemesoporous structure (pore size: 2e50 nm)
are introduced in this present work to increase ion absorption
properties through its unique entanglement network onto the
bigger pores of carbon [4]. CNTs also possess superb properties
such as superior mechanical stability, excellent electrical
properties, high dimensional ratios, low mass density, high
chargeedischarge capability and better chemical stability
with well-defined hollow core shape [5e7].
Electrolyte is also a crucial component in EDLCs. Solid state
polymer electrolytes have been widely investigated to replace
liquid electrolytes since they are prone to solve the leakage
problems. Solid polymer electrolytes have a wide range of
applications, ranging from small scale production of com-
mercial secondary lithium ion batteries (also known as the
rechargeable batteries) to advanced high energy electro-
chemical devices, such as chemical sensors, fuel cells, elec-
trochromic windows (ECWs), solid state reference electrode
systems, supercapacitors, thermoelectric generators, analog
memory devices and solar cells [8,9]. Armand and co-workers
review the newest aspects of ionic liquids in applications, for
example, as electrochemical solvents for metal/semi-
conductor electrodeposition and as conventional media to
replace as organic solvents in batteries or water batteries and
in polymer electrolyte membrane fuel cells (PEMFCs) [10].
Fisher et al. prepared solid hybrid polymer electrolyte based
on tri-ethyl sulfonium bis(trifluorosulfonyl) imide (S2TFSI),
lithium TFSI, and poly(ethylene oxide) (PEO) for lithium bat-
teries. The fabricated battery possesses reversible cathodic
stability exceeding 4.5 V and long term cycling stability
against metallic lithium [11]. Ionic liquid-added polymer
electrolyte-based dye-sensitized solar cell (DSSC) is also
assembled by Singh et al. (2009). Polymer electrolyte films
based on poly(ethylene oxide) and ionic liquid, 1-methyl 3-
propyl imidazolium iodide (PMII) were prepared by solution
casting technique and investigated. A DSSC based on the
highest conducting PEO:PMII/I2 electrolyte showed an energy
conversion efficiency of 0.81% at 100 mW cm�2 [12].
Poly(vinyl alcohol) (PVA) based-proton conducting polymer
electrolytes are employed in this present work. PVA-ammo-
nium acetate proton conductors have been widely investi-
gated [13,14]. However, these proton conductors are not
applicable in any electrochemical devices due to their low
ionic conductivity. Doping of ionic liquid is one of the ways to
enhance the ionic conductivity of polymer electrolytes. Ionic
liquid has several attractive characteristics, such as high ion
content, better thermal stability, non-volatile, non-flam-
mable, low viscosity, wider electrochemical operating poten-
tial window as well as environmentally friendly [15]. This
present work reports the effect of ionic liquid on the polymer
electrolytes and the electrochemical performances of the
fabricated EDLCs. The electrochemical properties of the
assembled EDLC are also studied.
Experimental
Materials
Polymer electrolytes containing PVA, CH3COONH4 and 1-
butyl-3-methylimidazolium bromide (BmImBr) were prepared
in this work. PVA (SigmaeAldrich, USA, 99% hydrolyzed with
molecular weight of 130,000 g mol�1), CH3COONH4 (Sigma,
Japan) and BmImBr (Merck, Germany) were used as host
polymer, salt and ionic liquid, respectively. All the materials
were used as received.
Preparation of ionic liquid added poly(vinyl alcohol)-basedpolymer electrolytes
Poly(vinyl alcohol) polymer electrolytes were prepared by
means of solution casting. PVA was initially dissolved in
distilled water. Appropriate amount of CH3COONH4 was sub-
sequently mixed in PVA solution. The weight ratio of
PVA:CH3COONH4 was kept at 70:30. Different mass fraction of
BmImBr was then doped into the PVAeCH3COONH4 aqueous
solution to prepare ionic liquid added polymer electrolytes.
The resulting solution was stirred thoroughly and heated at
70 �C for a few hours. The solution was eventually cast in a
glass Petri dish and dried in an oven at 60 �C to obtain a free-
standing polymer electrolyte film.
Characterization of ionic liquid added poly(vinyl alcohol)-based polymer electrolytes
Ambient temperatureeionic conductivity studiesFreshly prepared samples were subjected to aceimpedance
spectroscopy for ionic conductivity determination. A digital
micrometer screw gauge was used to measure the thickness
of the samples. The impedance of the polymer electrolytes
was measured using the HIOKI 3532-50 LCR HiTESTER
impedance analyzer over the frequency range between 50 Hz
and 5 MHz at ambient temperature. The measurement was
taken by sandwiching the polymer electrolyte between two
stainless steel (SS) blocking electrodes at a signal level of
10 mV. The ionic liquid-free and the highest conducting ionic
liquid added polymer electrolytes were subjected to the linear
sweep voltammetry (LSV) study and EDLC fabrication.
Differential scanning calorimetry (DSC)DSC analysis was performed using the TA Instrument Univer-
sal Analyzer 200 which consists of a DSC Standard Cell FC as
main unit and Universal V4.7A software. The whole analysis
was analyzed in a nitrogen atmosphere at a flow rate of
60 ml min�1. Samples weighing 3e5 mg were hermetically
sealed in an aluminum Tzero pan. A tiny hole was punched on
top of the pan to eliminate the water and moisture which are
released in the heating process. In contrast, an empty
aluminum pan was hermetically sealed as reference cell. The
samples were heated from 25 �C to 105 �C at a heating rate of
10 �Cmin�1 to remove any trace amount of water andmoisture
as a preliminary step. The heating process was maintained at
105 �C for 5min to ensure the complete evaporation. After that,
an equilibrium stage was achieved at 25 �C. The samples were
thus heated from 25 �C to 200 �C and followed up with a rapid
cooling process to �70 �C at the pre-set heating rate. The
sampleswere eventually reheated to 230 �C at the sameheating
rate. Crystallization temperature (Tc) was obtained in the
cooling cycle. On the other hand, glass transition temperature
(Tg) and crystalline melting temperature (Tm) were evaluated
using the final heating scan with the provided software.
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 0 ( 2 0 1 5 ) 8 5 2e8 6 2854
Thermogravimetric analysis (TGA)TGA was carried out using a thermogravimetric analyzer, TA
Instrument Universal Analyzer 2000 with Universal V4.7A
software. Samples weighing 2e3 mg were placed into a 150 ml
silica crucible. The samples were then heated from 25 �C to
600 �C at a heating rate of 50 �C min�1 in a nitrogen atmo-
sphere with a flow rate of 60 ml min�1.
Linear sweep voltammetry (LSV)CHI600D electrochemical analyzer was used to evaluate LSV
responses of ionic liquid-free polymer electrolyte and the
most conducting ionic liquid added polymer electrolyte. These
cells were analyzed at a scan rate of 10 mV s�1 by placing the
polymer electrolyte between SS electrodes in the potential
range of ±3 V.
Electrodes preparation
Activated carbon-based EDLC electrodes were prepared by
dip coating technique. The preparation of carbon slurry was
prepared by mixing 80 wt.% activated carbon (Kuraray
Chemical Co Ltd., Japan) of particle size between 5 and 20 mm,
surface area between 1800 and 2000 m2 g�1, 5 wt.% carbon
black (Super P), 5 wt.% multi-walled carbon nanotubes (CNTs)
(Aldrich, USA) with outer diameter, O.D. between 7 and
15 nm and length, L ranging from 0.5 to 10 mm) and 10 wt.%
poly(vinylidene fluoride) (PVdF) binder (molecular weight of
534,000 g mol�1 from Aldrich) and dissolving them in 1-
methyl-2-pyrrolidone (Purity � 99.5% from Merck, Germany).
Activated carbon was initially treated with sodium hydroxide
(NaOH) and sulfuric acid (H2SO4) to increase the porosity of
carbon. This slurry was stirred thoroughly for several hours
at ambient temperature. The carbon slurry was then dip
coated on an aluminum mesh current collector. The coated
electrodes were dried in an oven at 110 �C for drying
purposes.
EDLC fabrication
EDLC cell was constructed in the configuration of electrode/
polymer electrolyte/electrode. The EDLC cell configuration
was eventually placed in a cell kit for further electrochemical
analyses.
EDLC characterization
The fabricated EDLC cell was subsequently subjected to cyclic
voltammetry (CV) and galvanostatic chargeedischarge (GCD).
The cell using the ionic liquid-free polymer electrolyte
(denoted as BR 0) is classified as type I EDLC, whereas the one
using the most conducting ionic liquid added polymer elec-
trolyte (BR 6) as type II EDLC.
Cyclic voltammetry (CV)The CV study of EDLC was investigated using CHI600D elec-
trochemical analyzer. The cell was rested for 2 s prior to the
measurement. The EDLC cell was then evaluated at 10 mV s�1
scan rate in the potential range between 0 and 1 V in intervals
of 0.001 V. The specific capacitance (Csp) of EDLC was
computed using the equation as follows [16,17]:
Csp ¼ ism
�F g�1
�(1)
Csp ¼ isA
�F cm�2
�(2)
where i is the average anodicecathodic current (A), s is the
potential scan rate (V s�1), m refers to the average mass of
active materials (including the binder and carbon black) and A
represents surface area of the electrodes, that is 1 cm�2. The
average mass of electrode materials is around 0.01 g.
Electrochemical impedance spectroscopy (EIS)The impedance of the EDLC was probed by a HIOKI 3522-50
LCR HiTESTER impedance analyzer at room temperature with
a bias voltage of 10 mV. The EIS measurements were done in
the frequency range from 10 mHz to 100 kHz. The capaci-
tances, C were determined from the impedance data at a
frequency of 10 mHz using the following equation [18]:
C ¼ � 1uZ00 ¼ � 1
2pf � Z00 (3)
where u is angular frequency, which is represented by 2pf and
Z00 is the imaginary part of the complex impedance (Z). The
specific capacitance of EDLC was calculated by dividing the
capacitance with average weight of active materials. The
average weight of electrode materials is 0.02 g.
Galvanostatic chargeedischarge performance (GCD)The chargeedischarge study was carried out using a Neware
battery cycler. EDLC was charged and discharged at current of
1 mA. EDLC was allowed to rest for 10 min before taking the
measurements. The specific discharge capacitance (Csp) was
obtained from chargeedischarge curves, according to the
following relation [17]:
Csp ¼ I
m
�dV=dt
� (4)
where I is the applied current (A), m is the average mass of
electrode materials (including the binder and carbon black),
dV represents the potential change of a discharging process
excluding the internal resistance drop occurring at the
beginning of the cell discharge and dt is the time interval of
discharging process. The dV/dt is determined from the slope
of the discharge curve. The mass of the electrode used in this
study is 0.02 g.
Energy density, E (W h kg�1), power density, P (kW kg�1)
and Coulombic efficiency, h (%) were assessed using the
equations below [19]:
E ¼ Csp � ðdVÞ22
� 10003600
(5)
P ¼ I� dV2�m
� 1000 (6)
h ¼ tdtc
� 100 (7)
where td and tc are the discharging and charging times,
respectively.
i n t e r n a t i o n a l j o u rn a l o f h y d r o g e n en e r g y 4 0 ( 2 0 1 5 ) 8 5 2e8 6 2 855
Results and discussion
Ambient temperatureeionic conductivity studies
Fig. 1 portrays the ionic conductivity of polymer electrolytes
with respect to different mass loadings of BmImBr.
The ionic conductivity of polymer electrolytes increases
with the concentration of ionic liquid, up to a maximum level
after which conductivity decreases on further increase of IL
concentration. The ionic conductivity of polymeresalt elec-
trolyte was enhanced by two orders of magnitude, from
(1.94 ± 0.01) � 10�5 S cm�1 to (9.29 ± 0.01) � 10�3 S cm�1 with
addition of 60 wt.% of BmImBr (designated as BR 6). The
increment of ionic conductivity is related to the strong plas-
ticizing effect of ionic liquid. This effect not only softens the
polymer backbone, but it also helps in producing sticky poly-
mer electrolytes. The softening of polymer matrix could pro-
mote the dissociation of charge carriers (or ions) byweakening
the coordinative bonds and hence lead to rapid ionic con-
duction. On the other hand, the sticky behavior of polymer
membrane can provide better electrodeeelectrolyte contact.
This feature is very vital in any fabrication of electrochemical
device, especially in EDLCwhere its energy storage arises from
the charge accumulation between electrode and electrolyte
interface. The inherent physicochemical properties of ionic
liquid, i.e. low viscosity and high dielectric constant can be the
contributors for high ionic conductivity in the ionic liquid
added polymer electrolytes [3]. Low viscosity of ionic liquid
could produce highly flexible polymer chains and thus im-
proves the ionic mobility of the mobile charge carriers. In
contrast, high dielectric constant can shield the cationeanion
interaction in the polymer matrix and hence help in dissoci-
ating the cations from the attractive bonding with anions [3].
As a result, high dielectric constant promotes charge carrier
concentration.
Ionic liquid is also an additive to improve the amorphous
region. We suggest that ionic liquid could break the coordi-
nation bonds among the molecules and hence disrupt the
ordered chain structure. As a result, disordered arrangement
of macromolecules with a random coil configuration is ob-
tained. However, the polymer electrolytes become less
Fig. 1 e The ionic conductivity of polymer electrolytes with
different weight fraction of BmImBr.
conductive at highmass fraction of ionic liquid. Formations of
ion pairs and ion aggregates contribute to this phenomenon.
These ion pairs and ion aggregates would impede the ionic
transportation within the polymer electrolytes. The ion
transport mechanism is similar to previously reported work
[20]. The imidazolium cation (BmImþ) and bromide (Br�) areinitially detached from the transient partial bonding of ionic
liquid due to the bulky size of the cation. The hydrogen at C2-
position of the mobile BmImþ is then de-protonated to form a
stabilized carbene [20]. Thus, the produced carbene interacts
with the hydrogen in hydroxyl group of PVA and results car-
bocation in the imidazolium ring. The dissociated bromide
and acetate anions from ionic liquid and ammonium salt will
interact with the carbocation and hence break the hydrogen
bond between the imidazolium cations and side chain of PVA.
The oxygen atom in the hydroxyl group of PVA becomes
negatively charged (or anions) due to the removal of hydrogen
bond. Therefore, these electron deficient oxygen anionswould
accept the electron from proton from the loosely bounded
ammonium cations or from imidazolium cations and this is
the place to generate the proton transport mechanism in the
polymer electrolytes.
Differential scanning calorimetry (DSC)
Thermal behavior of polymer electrolyte is analyzed by DSC.
Typical DSC thermogram of BR 2 is displayed in Fig. 2. Two
temperatures are found in the heating curve, whereas there is
only one temperature in the cooling part, as can be seen in
Fig. 2.
The initial drop in heat flow in the heating scan is known as
glass transition temperature (Tg). The Tg is defined as the
temperature at where a glassy state of polymer is changed to a
rubbery phase in the amorphous region. BR 2 shows the Tg of
16.1 �C where the respective onset and endset temperatures
are 7 �C and 24.4 �C. The heat of flow decreased insignificantly
until an endothermic peak was found at 175.9 �C. This endo-
thermic peak is the crystalline melting temperature (Tm). At
this stage, the polymer could lose its elastomeric properties
and bemelted into a flowable liquid upon further heating. The
molecule transition at this temperature is changed from a
rubbery phase to a melt state by providing more energy to the
macromolecules. On the other hand, the exothermic peak at
135.5 �C in the cooling process is designated as crystallization
temperature (Tc). The macromolecules will arrange them-
selves spontaneously into a crystalline form when the poly-
mer is cooled down rapidly. A small drop in heat flow is
attained thereafter in the cooling scan and it is suggestive of
Tg.
Table 1 summarizes the Tg, Tm and Tc of pure PVA, BR 0, BR
2, BR 4 and BR 6, respectively.
These temperatures are decreased upon inclusion of
ammonium salt and ionic liquid. The Tg of pure PVA is around
80.15 �C. The Tg of the polymer membrane decreased to
46.58 �C with addition of salt as reported in our published
paper [21]. Similarly, upon doping with ionic liquid, Tg of
polymer electrolyte decreased from 46.58 �C to 16.61 �C (BR 2)
and 6.21 �C (BR 4), as illustrated in Table 1. The Tg of the most
conducting polymer electrolyte is reduced to sub-ambient
temperature of �1.38 �C. Low Tg infers high flexibility of the
Fig. 2 e Typical DSC curve of BR 2 at cooling and heating scans.
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 0 ( 2 0 1 5 ) 8 5 2e8 6 2856
polymer chains. The reduction of Tg is primarily due to the
strong plasticizing effect of ionic liquid. The plasticizing effect
weakens the polymer chains and thus improves the flexibility
of the polymer electrolytes. This feature helps in promoting
the ionic transportation in the polymer electrolytes. The same
trend has also been observed in Tm and Tc, as shown in Table
1. Both of these temperatures exhibit downward shift. Upon
inclusion of dopant salt, the Tm is reduced from 216.87 �C to
199.25 �C, whereas the Tc is decreased from 188.93 �C to
156.72 �C. These temperatures are also shifted to lower tem-
perature when we add the ionic liquid into the polymer sys-
tem. BR 2 exhibits an endothermic peak at 177.88 �C, whereas
BR 4 shows lower Tm at 117.43 �C. On the other hand, BR 2 and
BR 4 portray Tc at 135.54 �C and 71.20 �C, respectively.The relative degree of crystallinity is also calculated from
the melting endotherm using the equation below:
Xc ¼ DHm
DHqm
� 100% (8)
where DHm denotes the heat of fusion of sample and DHqm is
the heat of fusion of pure PVA obtained from the DSC result
(662.2 J g�1 in this work). The heat of fusion is the area under
the curve of melting peak which can be determined using the
Table 1 e The heat of fusion, relative crystallinity, glass transiand crystallization temperature (Tc) of pure PVA, BR 0 and ioni
Sample Heat offusion (J g�1)
Relativecrystallinity (%)
Glass trantemperature
Pure PVA 662.2 100.00 80.15
BR 0 621.5 93.85 46.58
BR 2 569.4 85.99 16.61
BR 4 140.5 21.22 6.21
BR 6 e e �1.38
Universal V4.7A software. The percentage of crystallinity of
ionic liquid-free polymer electrolyte is reduced slightly
compared to pristine PVA. This observation denotes that the
impregnation of ammonium salt not only produces flexible
chain, but also decreases the degree of crystallinity of the
polymer electrolytes. For ionic liquid added polymer electro-
lytes, the degree of crystallinity is expected to be lowered as
we deduced that doping of ionic liquid can reduce the crys-
talline portion of the polymer electrolytes. This can be proven
from the DSC thermogram. Addition of 20 wt.% of BmImBr
decreased the degree of crystallinity moderately to 86%.
However, the percentage of crystallinity is reduced abruptly to
around 21%with further addition of 20 wt.% of ionic liquid. All
samples depict both Tm and Tc, except BR 6. This important
feature denotes the semi-crystalline characteristic of all
polymer systems, except BR 6. BR 6 does not show any Tm and
Tc within the temperature regime, as revealed in Fig. 3.
The absence of these temperatures infers that BR 6 is
almost totally amorphous. Therefore, we can conclude that
the ionic migration of BR 6 takes place in the amorphous
phase. Moreover, the Tg obtained from the cooling curve is
comparable with the Tg in the heating scan. As a result, the Tg
is reversible in both curves.
tion temperature (Tg), crystalline melting temperature (Tm)c liquid added polymer electrolytes.
sition(Tg) (�C)
Crystalline meltingtemperature (Tm) (�C)
Crystallizationtemperature (Tc) (�C)
216.87 188.93
199.25 156.73
177.88 135.54
117.43 71.20
e e
Fig. 3 e Typical DSC curve of BR 6.
i n t e r n a t i o n a l j o u rn a l o f h y d r o g e n en e r g y 4 0 ( 2 0 1 5 ) 8 5 2e8 6 2 857
Thermogravimetric analysis (TGA)
Fig. 4 describes the TGA curves of PVA, BR 0, BR 2, BR 4 and
BR 6.
The TGA curve of PVA is not explained in this section as it
had been reported in our previous paper [20]. Four degradation
steps are observed in all ionic liquid added polymer electro-
lytes. The initial weight loss is assigned to the evaporation of
water, elimination of trapped moisture and removal of im-
purities. BR 0, BR 2, BR 4 and BR 6 exhibit the respective mass
losses of 5%, 9%, 7% and 8% in the temperature range of
25e150 �C. The mass of polymer electrolytes remains stable
above this dehydration stage until an abrupt drop in mass is
observed subsequently. BR 0 and BR 2 start to decompose at
240 �C and 250 �C with mass losses of 32% and 47%, respec-
tively. However, the degradation temperature range of BR 4
and BR 6 has been extended to 260e355 �C and 275e360 �C.We
Fig. 4 e Thermogravimetric analysis of pure PVA, BR 0 and
ionic liquid added polymer electrolytes.
take note that the degradation temperature of BR 6 is 20 �Cthan the most conducting BmImCl-based polymer electrolyte
which is 250 �C as reported in our published work [20]. This
observation denotes bromide based-polymer electrolytes
have better thermal stability than chloride based-polymer
electrolytes which is an important key in the safety perfor-
mance of the solid state electrochemical devices. The mass
loss at this stage also increased to 54% and 59% for BR 4 and BR
6, respectively. This is attributed to the decomposition of PVA
and ammonium acetate. The same degradation mechanism
stated in previously published work is used to explain the
decomposition process in each stage [20]. Ether cross-linkages
between the macromolecules could be formed as a result of
water elimination. The chain stripping process on these cross-
linkages can remove the side chain of PVA and induce weight
loss at this stage [20]. Besides, we suggest that the weight loss
is attributed to the degradation of acetamide (CH3C(O)NH2),
which is formed by the dehydration of ammonium acetate.
Since Lee et al. [22] reported that the decomposition temper-
ature of BmImBr is 252 �C,we imply that the cause of thismass
loss is due to the decomposition of BmImBr in the ionic liquid
added polymer electrolytes. This idea is supported by higher
mass loss obtained in thermogram of ionic liquid added
polymer electrolytes than BR 0 and pure PVA. It is also noted
that the degradation temperature of ionic liquid added poly-
mer electrolytes is slightly higher than pure PVA and the ionic
liquid-free polymer electrolyte. This is indicative of the
complexation between PVA, ammonium acetate and BmImBr
as higher energy is required to break these interactive bonds.
Beyond this mass loss, it is followed by another two
gradual drops in mass are observed. BR 0 displays around 20%
of mass loss from 245 �C until 375 �C, whereas BR 2 shows 11%
mass loss and its degradation temperature regime is between
345 and 420 �C. BR 4 and BR 6 have respective mass losses of
13% and 12% within the degradation temperature of
355e440 �C. The bromide-based polymer complexes show
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 0 ( 2 0 1 5 ) 8 5 2e8 6 2858
excellent thermal stability in comparison to chloride-based
polymer complexes as aforementioned in previous paragraph.
This can be proven again in this decomposition stage. The
most conducting chloride-based polymer electrolyte displays
the weight loss of 26% in the temperature range of 305e355 �C.Since bromide system possesses lower weight loss and higher
temperature compared to chloride system, we can conclude
that bromide system is a better choice as polymer electrolyte
in the electrochemical devices in terms of thermal stability.
The latter mass loss is the final weight loss before the samples
have been fully decomposed. BR 0 and BR 2 start to lose 27%
and 25%mass, along with 11% and 8% residual mass at 375 �Cand 410 �C respectively. Upon further addition of ionic liquid,
the mass loss in the final stage is improved. BR 4 exhibits
around 19% of mass loss with the remaining mass of 7% at
440 �C. Mass loss of 14% with 5% of residue is observed for BR
6 at 435 �C. These two mass losses are strongly related to
chemical degradation processes in the polymer chains such as
random chain scissoring between carbonecarbon bonds and
disruption of double bond in polyene of the polymer backbone
[20]. The mass of the polymer system remains stable above
550 �C. This finding infers the complete decomposition of the
polymer membrane. BR 6 is a promising candidate as polymer
electrolyte as it achieves the highest first degradation
temperature.
Linear sweep voltammetry (LSV)
The LSV curves of ionic liquid-free polymer electrolyte
(designated as BR 0) and the most conductive polymer elec-
trolyte (assigned as BR 6) are shown in Fig. 5 (a) and (b),
respectively.
(a)
(b)
Fig. 5 e (a): LSV response of BR 0. (b): LSV response of BR 6.
The potential window range of BR 0 is around 3.3 V, starting
from �1.6 V to 1.7 V. However, the operational potential
window of ionic liquid added polymer electrolyte is wider. The
cell can be charged up to 3.8 V in the regime between �1.8 V
and 2 V. This observation reveals that the ionic liquid doping
can improve the electrochemical stability window of the
polymer matrix. The operational current of ionic liquid-added
polymer electrolyte is enormously higher than ionic liquid-
free polymer electrolyte. We suggest that it is related to higher
ionic conductivity of polymer electrolytes. High ion concen-
tration in the polymer electrolytes and rapid ion transport
mechanism could lead to higher operational current in the
ionic liquid-added polymer electrolyte. These mobile charge
carriers are transported easily from one stainless steel elec-
trode to the other. The ease of ion transportation would lead
to more charge accumulation at the electrodeeelectrolyte
boundary. So, more electrons are required to generate the
current in the circuit.
Cyclic voltammetry (CV)
The electrochemical behavior of fabricated EDLC is inspected
using CV study. Fig. 6 (a) and (b) depicts cyclic voltammetries
of type I and type II EDLCs, respectively.
Fig. 6 e (a): Cyclic voltammograms of type I EDLC. (b): Cyclic
voltammograms of type II EDLC.
(a)
(b)
Fig. 7 e (a): Nyquist impedance plot of type I EDLC at room
temperature from 10 mHz to 100 kHz with close-up view of
the plot in high frequency region (inset) and its fitted data.
(b): Nyquist impedance plot of type II EDLC at room
temperature from 10 mHz to 100 kHz with close-up view of
the plot in high frequency region (inset) and its fitted data.
i n t e r n a t i o n a l j o u rn a l o f h y d r o g e n en e r g y 4 0 ( 2 0 1 5 ) 8 5 2e8 6 2 859
Redox peak from a Faradic current is absent in both figures
inferring the non-Faradic reaction in the EDLC. In other words,
the energy storage of the EDLC is based on the ion absorption
at the electrodeeelectrolyte interface without presence of any
chemical reaction. Type I EDLC shows a leaf-like shape CV
curve with specific capacitance of 0.14 F g�1 (or equivalent to
0.0015 F cm�2) in Fig. 6 (a). However, the specific capacitance of
type II EDLC has been enhanced drastically upon inclusion of
ionic liquid into the polymer electrolyte. The CV curve of type
II EDLC demonstrates a voltammogram approaching ideal
box-like shape with specific capacitance value of 21.89 F g�1
(or equivalent to 0.2650 F cm�2). This increment of around
15,535% in specific capacitance is owing to the high ionic
conductivity of polymer electrolyte as a result of plasticizing
effect and high ion content of ionic liquid, as mentioned in
Section 3.1. For a conductive polymer electrolyte, the amount
of mobile ions transporting within the medium could be
higher with enhanced mobility. This theory explains why
ionic liquid added polymer electrolytes have higher specific
capacitance. Therefore, more free ions are drifted from an
electrode to another electrode and hence absorbed onto the
carbon pores forming charge accumulation at the electro-
deeelectrolyte region. This charge accumulation is well-
known as electrical double layer. The energy could be stored
when the voltage is applied across the circuit.
Moreover, better electrodeeelectrolyte contact in the ionic
liquid added polymer electrolyte is another reason causing
higher capacitance in type II EDLC. These mobile ions require
lower energy barrier to overcome the resistance of forming ion
absorption at the interface when the contact between elec-
trode and electrolyte is intimate. Consequently, the ions are
more easily to be absorbed onto the carbon-based electrodes.
This effect promotes the formation of electrical double layer
and ultimately leads to increase in capacitive behavior of
EDLC. Similar work is also reported in Pandey et al. [23]. Pan-
dey and his co-workers constructed EDLC using ionic liquid
added poly(ethylene oxide) polymer electrolytes and multi-
walled carbon nanotubes electrodes. Comparing our current
project with this literature, the result is almost 10 times lower
than our work, where the specific capacitance is just about
2.6e3 F g�1. So, it can be concluded that ionic liquid doped
PVA-based polymer electrolyte is a promising candidate as
one of EDLC components.
Electrochemical impedance spectroscopy (EIS)
EIS is also probed to determine the specific capacitance of
EDLC. Fig. 7 (a) and (b) illustrate the EIS impedance plot of
EDLC containing ionic liquid-free polymer electrolyte and the
most conducting ionic liquid added polymer electrolyte,
respectively along with its Randle's equivalent circuit. On the
contrary, the semicircular region in the impedance plot is
enlarged and displayed in inset of figures.
Both figures show similar pattern of plot. Two obvious
parts have been observed in both Nyquist impedance plots
within the frequency regime:
i) A straight line (more commonly known as spike) with less
than 45� at low frequency end
ii) A semicircle at high frequency end
Wepropose an equivalent circuit for the EDLCs as shown in
the inset of both figures. The experimental data was fitted and
stimulated with this proposed equivalent circuit using
ZSimpWin software. The simulation findings are listed in
Table 2.
The experimental data is well-fitted with the simulation
data as shown in Fig. 7. This linear steep rising curve with the
phase angle of ~45� indicates the presence of Warburg
Table 2 e Simulation results of equivalent circuitelements in EDLCs from the fitted EIS data.
Element Type IEDLC
Type IIEDLC
Bulk resistance, Rb (U) 753 5.5
Double layer capacitance, Cdl (mF) 0.02 29.6
Charge transfer resistance, Rct (U) 245 1.6
Warburg impedance, Wo (S.s5) 1.3 � 10�3 0.06
Constant phase element, CPE (S.sn) 1.1 � 10�3 0.057
Frequency power, n (0 < n < 1) 0.43 0.45
Fig. 8 e Galvanostatic chargeedischarge performances of
type II cell over first 5 cycles.
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 0 ( 2 0 1 5 ) 8 5 2e8 6 2860
impedance (Wo) in the equivalent circuit as shown in both
figures. This spike represents the capacitance of double layer
(Cdl) in the circuit. Again, existence of spike proves the
capacitance arising solely due to formation of electrical dou-
ble layer at the boundary of electrodeeelectrolyte due to the
polarization effect. The ions would be diffused in the elec-
trolyte and adsorbed onto the pores of porous electrode to
create the electrical double layer [23].
Two resistances of the electrochemical cells are attained in
the Nyquist impedance plot and its equivalent circuit, viz.
bulk resistance (Rb) of the cell and charge transfer resistance
(Rct) of ion diffusion to form the ion adsorption. The initial
resistance observed at high frequency end is well-known as
bulk resistance (Rb). This resistance arises from the bulk
resistance of the polymer electrolyte, series resistance (Rs) of
the connector and internal resistance of electrode as well as
ohmic loss [3,24]. In addition, the semicircle region is con-
structed of a parallel combination of capacitor and resistor, as
depicted in equivalent circuitmodel. The capacitance refers to
the bulk properties of the polymer electrolytes and interfacial
contact capacitance between porous carbon and mesh elec-
trode [16,23,25]. The capacitor is represented by the capaci-
tance of double layer (Cdl). The Cdl comes from the formation
of electrical double layer at electrodeeelectrolyte due to the
ion accumulation between electrode and electrolyte when the
ions are diffused into the porous carbon electrode. On the
contrary, the resistance denotes to the charge transfer resis-
tance (Rct) on the charge absorption onto the electrodes. So,
the ions must overcome this resistance in order to form the
electrical double layer. The intercept of the semicirclewith the
spike gives rise to the total internal resistance of the cell
which is the combination of Rb and Rct. So, Rct is calculated by
deducting the total resistance with Rb. The Rb values obtained
in type I and type II EDLCs are 680 U and 5.5 U, respectively. On
the other hand, the respective Rct values of type I and type II
EDLCs are 245 U and 1.6 U. The resistance of type II EDLC is
relatively lower than that of type I EDLC. Sticky behavior of the
ionic liquid added polymer electrolyte is the main factor to
lower down the resistance in this phenomenon. This inherent
property could provide excellent interfacial contact between
electrode and electrolyte. The ions require lower energy to be
transported within the polymer matrix when the polymer
electrolyte possesses low resistance barrier. Rapid mass
transport within the pores of porous activated carbon based
electrode also decreases the charge transfer resistance [26].
Again, we observe that the Cdl and CPE of type II EDLC have
higher value than type I EDLC. These findings prove that
addition of ionic liquid can improve the ion diffusion in the
electrolyte and thus promote the ion adsorption at the elec-
trodeeelectrolyte boundary.
The specific capacitance of type I EDLC obtained in EIS is
around 0.13 F g�1. However, the specific capacitance of type II
EDLC has been increased abruptly to 21.63 F g�1 by doping
ionic liquid into the polymer electrolyte. The result is in good
agreement with CV findings. Conductive behavior of ionic
liquid added polymer electrolyte is the main contributor for
enhancing the capacitance of EDLC, as explained in previous
section. The charge carriers can be dissociated easily from the
polymer complex when ionic liquid is added into the matrix.
Thus, the polymer backbone could turn into flexible chain.
Therefore, the number of ions and ionicmobility are increased
thereafter which leads to higher ion absorption onto the
electrodes. Another reason of higher capacitance of type II
EDLC is the sticky and adhesive properties of ionic liquid
added polymer electrolyte which is probably due to strong
plasticizing effect of ionic liquid. Based on the result, it reflects
the effect of addition of ionic liquid on the capacitive behavior
of EDLC.
Galvanostatic chargeedischarge performance
Galvanostatic chargeedischarge is another tool to compute
the specific capacitance of EDLC. Fig. 8 shows the galvano-
static chargeedischarge performance of type II EDLC over 5
cycles of charging and discharging.
The starting cell potential of EDLC during charging process
is 0.15 V instead 0 V, meanwhile the cell potential starts at
0.85 V instead of 1 V for discharging process. These phenom-
ena are associated with the internal resistance of the cell.
Factors that cause the ohmic loss of the EDLC are interfacial
resistance between electrolyte and electrode, interfacial
resistance between current collector and active material, and
resistances of electrolyte, active materials and connector
[16,27]. It is noteworthy that the internal resistance of the cell
increases somewhat with cycle number, as described in Fig. 8.
We suggest that the ions might form the neutral ion pairs due
Fig. 9 e Specific capacitance and Coulombic efficiency of
type II EDLC over 500 cycles.
i n t e r n a t i o n a l j o u rn a l o f h y d r o g e n en e r g y 4 0 ( 2 0 1 5 ) 8 5 2e8 6 2 861
to the rapid charge adsorption onto the carbon at high cycle
number. Therefore, the mobile charge carriers transported
into the electrolyteeelectrode interface become lesser and
lead to the depletion of polymer electrolyte. In addition, these
ion pairs could block the ion passage in the electrolyte and
hence enhance the resistance of the cell [26].
The specific discharge capacitance, Coulombic efficiency,
energy density and power density of the electrochemical cell
obtained in the first cycle are 21.38 F g�1, 70%, 2.18 Wh kg�1
and 41.27 kW kg�1 respectively. The specific discharge
capacitance is similar with the results obtained in CV and EIS
studies. The electrochemical stability of the EDLC is further
analyzed by subjected the performance over 500 cycles. The
long-term cyclability tests of type II EDLC are revealed in Figs.
9 and 10.
The electrochemical properties of EDLC fade with
increasing the cycle number of charging and discharging
processes, as shown in both figures. There is a drastic drop in
specific discharge capacitance, energy density and power
density below 250 cycles of charging and discharging. Upon
charging and discharging for 250th cycles, the specific
discharge capacitance is reduced about 11 %e19.02 F g�1,
meanwhile power density exhibits around 15% of drop along
with the value of 35 kW kg�1. However, around 36% of
decrease in energy density is obtained, where its value is
Fig. 10 e Energy density and power density of type II EDLC
over 500 cycles.
1.40 Wh kg�1. The decrease in the electrochemical perfor-
mances is suggestive of depletion of electrolyte. In addition,
formation of neutral ion pairs is a possible contributor in
decreasing the electrochemical stability. Mobile charge car-
riers which are available for transportation from an electrode
to opposite electrode are reduced in the formation of ion pairs
and ion aggregates. Therefore, the ion absorption onto the
electrodes is thus reduced.
In contrast, the Coulombic efficiency of the cell remains in
the range of 70e89 % over 500 cycles. The cell remains almost
stable above 250 charging and discharging cycles. The cell
possesses specific discharge capacitance of 18.84 F g�1,
1.36 Wh kg�1 and 34.66 kW kg�1 are attained upon 500 cycles
of charge and discharge processes. So, we can conclude that
the prepared ionic liquid added polymer electrolyte is a
promising candidate as a separator in the EDLC as it still can
maintain its electrochemical stability over 500 cycles of charge
and discharge processes.
Conclusions
Doping of ionic liquid not only enhances the ionic conductivity
of polymer electrolytes but also improves the electrochemical
potential window of polymer electrolytes. Inclusion of 60 wt.%
of ionic liquid increases the ionic conductivity of polymer
electrolytes by two orders of magnitude, which is (9.29 ± 0.01)
mScm�1 at ambient temperature. Addition of ionic liquid re-
duces theTg,TmandTc. Ionic liquid addedpolymer electrolytes
showbetter thermal stability in comparison to ionic liquid-free
polymer electrolyte. The specific capacitance of the con-
structed EDLCs can also be increased by 15,535% with adul-
teration of ionic liquid. The specific capacitance of 21.89 F g�1
was obtained for EDLC containing the most conducting poly-
mer electrolyte as shown in CV curve. The result is in a good
agreement with EIS and chargeedischarge studies. The elec-
trochemical stability was also examined over 500 cycles. The
fabricated EDLC remains stable after charge and discharge for
250 cycles. Ionic liquid added polymer electrolyte is a potential
candidate as an electrolyte in EDLC.
Acknowledgment
This work was supported by the High Impact Research Grant
(UM.C/625/1/HIR/MOHE/SCI/21/1) from Ministry of Education,
Malaysia. One of the authors, Chiam-Wen Liew gratefully
acknowledges the “Skim Bright Sparks Universiti Malaya”
(SBSUM) for scholarship award.
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