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Good prospect of ionic liquid based-poly(vinylalcohol) polymer electrolytes for supercapacitorswith excellent electrical, electrochemical andthermal properties

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 29 November 2012

Received in revised form

26 April 2013

Accepted 14 June 2013

Available online 8 July 2013

Keywords:

PVA

Ionic liquid

Polymer electrolyte

Supercapacitor

Specific capacitance

* Corresponding author. Tel.: þ60 3 7967 439E-mail addresses: liewchiamwen85@gma

0360-3199/$ e see front matter Copyright ªhttp://dx.doi.org/10.1016/j.ijhydene.2013.06.0

a b s t r a c t

Poly(vinyl alcohol) (PVA)/ammonium acetate (CH3COONH4)/1ebutyle3emethylimi

dazolium chloride (BmImCl) based polymer electrolytes were prepared by solution casting

method. The ionic conductivity increased with temperature as shown in temperature

dependent-ionic conductivity study. The maximum ionic conductivity of (7.31 � 0.01)

mS cm�1 was achieved at 120 �C upon adulteration of 50 wt% of BmImCl. The samples

obeyed VogeleTammaneFulcher (VTF) relationship. The glass transition temperature (Tg)

of the polymer matrix was reduced by doping it with salt and ionic liquid as shown in

differential scanning calorimetry (DSC). Supercapacitor was thus assembled. Wider po-

tential stability range has been observed with addition of ionic liquid. Inclusion of ionic

liquid also improved the electrochemical behavior of EDLC. The capacitance of super-

capacitor were determined by cyclic voltammetry, electrochemical impedance spectros-

copy and galvanostatic chargeedischarge tester. The cell also illustrated energy density of

2.39 Wh kg�1 and power density of 19.79 W kg�1 with Coulombic efficiency above 90%.

Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction form transparent film [1]. PVA has merits of being a host

As the world paces towards green technology, the imple-

mentation of environmentally friendly materials in

manufacturing is an alternative way to reduce environmental

issues of plastic waste disposal throughout all phases of the

product life cycle. Therefore, biodegradable polymers have

received an upsurge of interest nowadays in replacing those

environmentally unfriendly polymers. Among biodegradable

synthetic polymer, poly(vinyl alcohol) (PVA) is a multitalented

material with excellent tensile strength and high ability to

1; fax: þ60 3 7967 4146.il.com (C.-W. Liew), rame2013, Hydrogen Energy P61

polymer due to its availability, good optical properties, cost

effectiveness, non-toxicity, high hydrophilicity and good

mechanical strength as well as high temperature resistance

[2,3]. PVA also possesses some inherent characteristics, viz.

biocompatible, ease of preparation, high abrasion resistance

and high flexibility with superior chemical and thermal sta-

bilities [4,5]. Other reasons for choosing PVA as host polymer

are greater extent of polar groups (hydroxyl group) and high

chain flexibility which promotes the salt-solvation [6]. High

dielectric constant, excellent charge storage capacity and

shtsubra@gmail.com (S. Ramesh), akarof@um.edu.my (A.K. Arof).ublications, LLC. Published by Elsevier Ltd. All rights reserved.

Table 1 e Nomenclature and weight percent of thematerials added in the ionic liquid basedePVA polymerelectrolytes.

Designations Weight percent ofPVAeCH3COONH4eBmImCl

PVA(wt%)

CH3COONH4

(wt%)BmImCl(wt%)

CL 0 70 30 0

CL 2 56 24 20

CL 5 35 15 50

CL 6 28 12 60

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 3 9 ( 2 0 1 4 ) 2 9 5 3e2 9 6 32954

dopantedependent electrical properties make PVA as a

promising candidate in the development of electrochemical

devices, especially EDLC and fuel cell applications [7].

Ammonium acetate is selected as doping salt in this current

project. Ammonium acetate (CH3COONH4) is chosen due to its

plasticizing effect. Besides, the acetate anion (CH3COO�) is

preferred in salt-solvation through hydrogen bonding.

The PVAeCH3COONH4 polymer system has been widely

investigated by G. Hirankumar and his co-workers [6e8].

However, the ionic conductivity is still too low and it is not

applicable in the electrochemical devices. Therefore, several

ways have been employed to improve the ionic conductivity,

such as polymer blending, addition of plasticizers, impreg-

nation of ionic liquids and doping of fillers. Among all the

methods, addition of ionic liquid is the most viable technique

to enhance the ionic conductivity because of its strong plas-

ticizing effect, environmental friendly nature and high ion

content. Ionic liquid is generally defined as a type of molten

salt composed solely of ions. Other features of ionic liquid are

excellent thermal stability, negligible vapor pressure, non-

volatile, low melting point, non-flammable, low viscosity,

wider electrochemical potential window and high ionic con-

ductivity [9]. The objective of this present work is to prepare

an environmentally friendly polymer electrolyte with superior

electrical, thermal and electrochemical properties. Another

aim is to investigate the effect of ionic liquid onto the polymer

electrolytes and supercapacitors. In this paper, we report the

effect of ionic liquid on the PVAeCH3COONH4 polymer elec-

trolytes in supercapacitor application. Supercapacitor (or

known as electrical double layer capacitor, EDLC) is fabricated

using the highest conducting polymer membrane in this

present work. Supercapacitor is the preferred choice

compared to secondary batteries owing to the longer cycle life

(>105 cycles), higher power density, higher capacitive density

and faster chargeedischarge rate as well as higher ability to be

charged and discharged continuously without degrading [10].

Fig. 1 e The image of ionic liquidebased PVA polymer

electrolyte.

2. Experimental

2.1. Materials

Polymer electrolytes containing PVA, CH3COONH4 and

1ebutyle3emethylimidazolium chloride (BmImCl) were pre-

pared in this current work. Synthetic biodegradable polymer,

PVA (SigmaeAldrich, USA, 99% hydrolyzed with molecular

weight of 130000 g mol�1), CH3COONH4 (Sigma, Japan) and

BmImCl (Acros organic, USA) were used as host polymer, salt

and ionic liquid, respectively. All the materials were used as

received.

2.2. Preparation of ionic liquid based-poly(vinyl alcohol)polymer electrolytes

Ionic liquid based-poly(vinyl alcohol) polymer electrolytes

were prepared bymeans of solution casting. Table 1 shows the

weight proportion of PVA, CH3COONH4 and BmImCl added

into the polymer electrolytes and the designations of the

polymer complexes. PVA was initially dissolved in distilled

water. Appropriate amount of CH3COONH4 was subsequently

mixed in PVA solution. The weight ratio of PVA:CH3COONH4

was kept at 70:30. Different weight ratio of BmImCl was thus

added into the PVAeCH3COONH4 mixture to prepare ionic

liquid-based polymer electrolyte. The resulting solution was

stirred thoroughly and heated at 70 �C for few hours until a

homogenous colorless solution was obtained. The solution

was eventually casted on glass Petri dish and dried in an oven

at 60 �C. A free-standing polymer electrolyte filmwas obtained

and shown in Fig. 1.

2.3. Characterization of ionic liquid based-PVA polymerelectrolytes

2.3.1. Temperature dependence-ionic conductivity studiesFreshly prepared samples were subjected to ac-impedance

spectroscopy for ionic conductivity measurements. Digital

micrometer screw gauge was used to determine the thickness

of the samples. The ionic conductivity of the polymer electro-

lytes was measured by HIOKI 3532-50 LCR HiTESTER imped-

ance analyzer over a frequency range between 50Hz and 5MHz

from ambient temperature to 120 �C at a signal level of 10 mV.

Polymer electrolytes were placed on the sample holder under

spring pressure in the configuration of stainless steel (SS)

blocking electrode/polymer electrolyte/SS electrode.

2.3.2. Differential scanning calorimetry (DSC)DSC analysis was carried out by TA Instrument Universal

Analyzer 200 which consists of DSC Standard Cell FC as main

i n t e r n 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 3 9 ( 2 0 1 4 ) 2 9 5 3e2 9 6 3 2955

unit and Universal V4.7A software. The whole analysis was

conducted in a nitrogen atmosphere with a flow rate of

60 ml min�1. Samples weighing 3e5 mg were hermetically

sealed in the aluminum Tzero pan, while, an empty

aluminum pan was hermetically sealed as reference cell. In

order to remove any trace amount of water and moisture, the

samples were heated from 25 �C to 105 �C at a heating rate of

10 �Cmin�1. The heating process wasmaintained at 105 �C for

5 min to ensure the complete evaporation. Beyond this step,

an equilibrium stage was achieved at 25 �C. After that, the

samples were heated from 25 �C to 150 �C and followed up

with a rapid cooling process to �50 �C at the pre-set heating

rate. The samples were eventually reheated to 150 �C at the

same heating rate. Glass transition temperature (Tg) was

evaluated using the final heating scan with the provided

software.

2.4. Electrodes preparation

The activated carbon-based EDLC electrodeswere prepared by

dip coating technique. The carbon slurry was prepared by

mixing 80 wt% of activated carbon (Kuraray Chemical Co Ltd.,

Japan, particle size is 5 w 20 mm, surface area is

1800 w 2000 m2 g�1), 10 wt% of carbon black (known as Super

P) and 10 wt% of poly(vinylidene fluoride) (PVdF) binder (mo-

lecular weight of 534000 g mol�1 from Aldrich) in 1-methyl-2-

pyrrolidone (Purity� 99.5% fromMerck, Germany). This slurry

was stirred thoroughly for several hours at ambient temper-

ature until homogenous slurry with smooth surface is ob-

tained. The aluminum electrode mesh and the slurry were

then subjected to dip coater for dip coating process. The speed

of lifting up and down is 90 mm s�1; meanwhile the duration

for the dipping process is 15 s. The coated electrodes were

dried in an oven at 70 �C for drying process. The weight of

electrode materials was evaluated including the binder and

super P.

2.5. EDLC fabrication

EDLC cell was fabricated by sandwiching the polymer elec-

trolyte between two symmetrical activated carbon-based

electrodes. The cell was then pressed by hydraulic hand

pump at 1MPa to ensure good contact between electrolyte and

electrode. The EDLC cell configuration was eventually placed

in a cell kit for further electrochemical analyses.

2.6. EDLC characterization

The EDLC cell was subsequently subjected to linear sweep vol-

tammetry (LSV), cyclic voltammetry (CV), electrochemical

impedance spectroscopy (EIS) and galvanostatic char-

geedischarge (GCD) tester for electrochemical characterization.

The effect of ionic liquid was investigated for all of these elec-

trochemical analysesbyassembling twodifferentEDLCs, except

GCD study. The cells using the polymer electrolyte without

doping of ionic liquid (denoted as CL 0) is designated as type I

supercapacitor, whereas the one using the most conducting

ionic liquid-based polymer electrolyte is assigned as type II

supercapacitor.

2.6.1. Linear sweep voltammetry (LSV)The LSV responses of CL 0 and CL 5 were accomplished using

CHI600D electrochemical analyzer. These cells were analyzed

at a scan rate of 5 mV s�1 with the configuration of stainless

steel (SS)/polymer electrolyte/SS in the potential range of

�3 V. The sample interval was 0.001 V with 2 s as the rest time

prior to the analysis.

2.6.2. Cyclic voltammetry (CV)CHI600D electrochemical analyzer was also used to scrutinize

CV measurement of EDLC. Initially, the cell was at rest for 2 s.

The EDLC cell was then evaluated at 10mV s�1 scan rate in the

potential range between 0 and þ1 V with sample interval of

0.001 V. The specific capacitance (Csp) of EDLC was calculated

using the following equation [11,12]:

Csp ¼ ism

(1)

where i is the average anodicecathodic current (A), s is the

potential scan rate (V s�1) and m is the average mass of active

materials (including the binder and super P). The average

mass of electrode materials is around 0.015 g.

2.6.3. Electrochemical impedance spectroscopy (EIS)The impedance spectroscopy was probed by HIOKI 3522-50

LCR HiTESTER impedance analyzer at room temperature with

a bias voltage of 10mV. The EISmeasurement was done in the

frequency range of 10 mHze100 kHz. The capacitance, C was

determined from the impedance data at a frequency of

10 mHz using the following equation [13]:

C ¼ � 1uZ00 ¼ � 1

2pf � Z00 (2)

where u is angular frequency, which is represented by 2pf and

Z00 is the imaginary part of the total impedance (Z ). The spe-

cific capacitance of EDLC was calculated by dividing the

capacitancewith averageweight of activematerials (including

the binder and super P). The average weight of electrode ma-

terials is 0.02 g.

2.6.4. Galvanostatic chargeedischarge performance (GCD)The chargeedischarge studywas performed using the Neware

battery cycler. The EDLC was charged and discharged at cur-

rent of 1 mA. EDLC is allowed to rest for 30 min prior to the

measurement. The specific discharge capacitance (Csp) was

obtained from chargeedischarge curves, according to

following relation [12]:

Csp ¼ I

m�dVdt

� (3)

where I is the applied current (A), m is the average mass of

electrode materials (including the binder and super P), dV rep-

resents 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 electrode used in this study is in 0.02 g.

Energy density (E, W h kg�1), power density (P, W kg�1) and

Coulombic efficiency (h, %) were assessed from the equations

below [14]:

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 3 9 ( 2 0 1 4 ) 2 9 5 3e2 9 6 32956

E ¼ Csp � ðdVÞ2 � 1000(4)

2 3600

P ¼ I� dV2�m

� 1000 (5)

h ¼ tdtc� 100 (6)

where td and tc are the discharging time and charging time,

respectively.

3. Results and discussion

3.1. Temperature dependence-ionic conductivity studies

Fig. 2 portrays the ionic conductivity of ionic liquid doped

polymer electrolytes and ionic liquid free-based polymer

electrolyte in the temperature regime between ambient tem-

perature and 120 �C. Upon addition of BmImCl, the ionic

conductivity is greatly enhanced by comparing CL 0 with CL 2,

CL 5 and CL 6. Strong plasticizing effect of ionic liquid is the

main contributor in this phenomenon [15]. The polymer

chains could be more flexible in the presence of ionic liquid.

So, the flexible polymer backbone increases the segmental

motion of the polymer matrix and hence assists the ionic

transportation which confers with high ionic conductivity.

Incorporation of ionic liquid also reduces the glass transition

temperature (Tg). Lower Tg would promote the rubbery phase

formation which constrains the crystalline degree of polymer

electrolytes in conjunction with higher amorphous propor-

tion. Higher amorphousness of polymer complexes initiates

the breakdown of transient coordination bonds between

charge carriers (or more preferably known as cation) and ox-

ygen from hydroxyl group in PVA. Therefore, more mobile

charge carriers can be migrated within the vicinity of the

unordered arrangement of the macromolecules. As a result,

higher ionic conductivity is obtained. Apart from that, the

ionic liquids help in deteriorating the coordinative bonding

between proton and the weakly bonded polar group in the

polymer chains. Consequently, the protons are free tomigrate

from one site to another site [9].

According to Kumar et al., the incorporation of ionic liquid

can alter the electrical properties in different ways, for

y = -1.1567x - 0.8177R² = 0.9009

y = -0.2946x - 1.9957R² = 0.9596

y = -0.1185x - 1.8214R² = 0.8872

y = -0.2852x - 2.1061R² = 0.9805

-5.00

-4.50

-4.00

-3.50

-3.00

-2.50

-2.00

2.5 2.6 2.7 2.8 2.9 3 3.1 3.2 3.3 3.4

log

[(

Scm

-1)]

1000/T (K-1)

CL 0 CL 2 CL 5 CL 6

σ

Fig. 2 e The Arrhenius plots of the ionic conductivity with

different mass fraction of BmImCl doped in the polymer

electrolytes over a wide temperature range.

example bigger size of anion will dissociate the cations easily

[16]. In this present work, the bulky group of BmImþ also helps

in improving the ionic conductivity by producing free volume

for conduction [16]. Moreover, the physicochemistry of ionic

liquid such as viscosity and dielectric constant also contrib-

utes to the increase in ionic conductivity. Lower viscosity of

ionic liquid also enhances the polymer chain flexibility.

Therefore, this flexible polymer backbone could interrupt the

ion-polymer bonding and promote the ionic conducting pro-

cess [16]. In contrast, high dielectric permittivity of ionic liquid

plays an important role to separate the ion pairs and/or ion

aggregateswith high self-dissociating properties. Moremobile

cations are consequently producedwhich leads to higher ionic

conductivity [16]. In addition, the ionic conductivity rises with

the ionic liquid concentration, as shown in CL 2 and CL 5 plots.

The same explanations as aforementioned also applied onto

this increase in ionic conductivity, such as increase in amor-

phous region and plasticizing effect of ionic liquid. The high-

est ionic conductivity of (7.31 � 0.01) mS cm�1 has been

achieved by adding 50 wt% of BmImCl at 120 �C. However, the

ionic conductivity is decreased with further incorporation of

BmImCl as shown in Fig. 2.

A moderate increase in ionic conductivity with a gradual

increase in temperature is observed in Fig. 2. This is principally

due to the fast vibrationmodes of themolecules in the polymer

membrane upon heating. Rapid vibration mode of the macro-

molecules promotes the bond rotation within the polymer

matrix. The cations coupling with higher polymer segmental

mobility can be detached from the coordination bond with the

oxygen in PVA easily at elevated temperature. Therefore, ionic

transportation is enhanced with increasing temperature. In

addition, the amplitude of the molecular vibration increases

with temperature. Therefore, the thermal vibration improves

the decoupling rate of cation from transient coordination

bonding with anion. More cations can be transported within

the polymer electrolytes with higher decoupling rate. Since the

number of mobile cations which is responsible for ionic

transportation is higher, thus more ionic conduction in the

polymer electrolytes would be generated. So, as expected,

higher ionic conductivity is attained in this study. Apart from

that, the polymer membrane could be expanded due to higher

amplitude of oscillation mode at elevated temperature. This

thermal expansion creates free spaces for conduction mecha-

nism. As a consequence, the charge carriers are decoupled and

hence transported easily, contributing to increase in ionic

conductivity with reciprocal of temperature.

Fig. 2 illustrates positively curved plot for all the samples

where its regression value is far away from unity [17]. These

non-linear variations reflect that all the polymer electrolytes

exhibit VogeleTammaneFulcher (VTF) theory which is asso-

ciated with free volumemodel [17,18]. In order to interpret the

result, the plots are then fitted with VTF empirical formula.

Based on this volume-activated principle, the ionic conduc-

tivity is expressed as below:

s ¼ AoT�12exp

� �BT� Tg

�¼ AoT

�12exp

��Ea=kB

T� Tg

�(7)

where Ao is pre-exponential constant proportional to the

number of charge carriers, B represents a constant which is

determined from the gradient of the plot (K�1), Ea is pseudo-

y = -0.09x - 2.3892R² = 0.9954

y = -0.0875x - 1.2712R² = 0.9929

y = -0.0825x - 0.6515R² = 0.9947

y = -0.0898x - 1.3087R² = 0.9908

-4.50

-4.00

-3.50

-3.00

-2.50

-2.00

-1.50

-1.00

5 10 15 20 25 30 35

log

[T

1/2

(Scm

-1K

1/2 )

]

1000/T-To((K-1)

CL 0 CL 2 CL 5 CL 6

σ

Fig. 3 e The temperature dependenteionic conductivity of

polymer electrolytes fitted with VTF relationship.

i n t e r n 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 3 9 ( 2 0 1 4 ) 2 9 5 3e2 9 6 3 2957

activation energy for conduction which is related to polymer

segmental mobility (eV), kB stands for Boltzmann constant

(8.6173 � 10�5 eVK�1), T represents the absolute temperature

(K) and To is ideal vitreous transition temperature at which

the polymer segments start to move [19]. Tg is also defined

as the temperature at zero configuration entropy of polymer

membrane and it is suggested to be 50 K below the glass

transition temperature (Tg) [20e22]. On the other hand,

experimental Tg is determined from DSC measurement. All

the plots are well-fitted with VTF rule where all the regres-

sion values are approximately to unity, as shown in Fig. 3.

Since all the regression lines approach to unity, therefore it

can be concluded that these polymer electrolytes follow VTF

behavior. VTF theory infers the cooperative process in the

ionic conduction mechanism where the ionic hopping

mechanism is coupled with higher polymer segmental

mobility in an amorphous phase [20]. This principle states

that the rise in temperature enhances the flexibility of

polymer chains as a result of increase in thermal oscillation

mode. The movement of polymer segments is thus

improved. Fast segmental mobility could promote the

breaking down of the weak interaction between proton and

polar group in the polymer. Therefore, the detachment of

the proton will create more free voids for the ions to jump

from an interstitial site to another adjacent equivalent site

and eventually generate the ionic transportation.

Anon-linear least square technique is carried out to evaluate

some output parameters (Tg, Ao, B and Ea) which are listed in

Table 2. In contrast, Tg is determined by subtracting experi-

mental Tg with 50 K. B is correlated with the rate at which the

viscosity of the sample changeswith respect to the temperature

[23]. This rate is hence replaced with the ratio of Ea and kB as

Table 2 e The obtained parameters from each VTF plot with ththermogram.

Samples Regressionvalue, R2

Ideal glass transitiontemperature, To (K)

Glass transitiontemperature, Tg (K)

CL 0 0.99 265.07 315.07

CL 2 0.99 223.03 273.03

CL 5 0.99 203.23 253.23

CL 6 0.99 213.78 263.78

displayed in equation (6). Ea of each sample is subsequently

calculated. As can be seen, the pseudo-activation energy is

inversely proportional to the ionic conductivity. Among all the

polymer electrolytes, CL 5 shows the highest ionic conductivity,

butbears the lowest pseudo-activationenergy. Thisobservation

reveals that CL 5 manifests higher flexibility of polymer back-

bone coupled with increased segmental mobility within the

polymerchains.Hence,CL5 requires loweractivationenergy for

the hopping process.

3.2. Differential scanning calorimetry (DSC)

DSC is a vital analysis to determine the glass transition tem-

perature (Tg) of the sample. This study signifies the phase

transition of a polymer matrix in the amorphous region, from

a hard glassy phase into a flexible and soft rubbery charac-

teristic. DSC thermograms of pure PVA, CL 0, and ionic liquid-

based polymer electrolytes are shown in Fig. 3. As aforemen-

tioned in introduction, ammonium acetate is a plasticizing

salt. This can be proven in DSC curve profiles. Pristine PVA

shows Tg of 80 �C. However, it is substantially reduced to

around 47 �C when 30 wt% of CH3COONH4 is complexed with

70 wt% of PVA. Tg of polymer membrane should be increased

upon addition of CH3COONH4 due to the partial coordination

bonding in the complexation. Nevertheless, an oppose result

is obtained. The significant decrease of Tg denotes that the

plasticizing effect of CH3COONH4 dominates the temporary

interactive coordination. This plasticizing effect softens the

polymer backbone and thus produces flexible polymer

backbone.

The Tg is decreased further with addition of ionic liquid.

Sub-ambient temperature of Tg is attained by adding ionic

liquid. These sub-ambient temperatures divulge the rubbery

state of polymer electrolytes as the surrounding temperature

is much higher than Tg. The molecules in the polymer matrix

are allowed to undergo orientation and conformational

changes in the rubbery phase. So, it can be concluded that the

chain flexibility of polymer electrolytes containing ionic liquid

is higher than the ionic liquid-free polymer complexes which

induces higher segmental mobility. The abrupt decrease in Tg

is assigned to the plasticizing effect of ionic liquid. Again, the

plasticizing effect stimulates the flexibility of polymer back-

bone. As expected, the ammonium salt would form a

complexation with the polymer chains. So, the charge carriers

which are responsible for the ionic transportation tend to be

dissociated from the native complexation owing to the flexible

polymer chains. As a result, the ionic transportation is

favored. In addition, Tg reduces with further inclusion of ionic

e experimental glass transition temperature from DSC

Preeexponentialconstant, Ao

Constant or gradientof the plot, B (K�1)

Pseudoeactivationenergy, Ea (meV)

2.3892 0.0900 7.76

0.2941 0.0875 7.54

1.2712 0.0825 7.11

0.6515 0.0898 7.74

-6

-4

-2

0

2

4

6

8

-4 -3 -2 -1 0 1 2 3 4

Cur

ent,

I (

µA) 1.8 V

–1.9 V

(a)

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 3 9 ( 2 0 1 4 ) 2 9 5 3e2 9 6 32958

liquid. The Tg exhibits downward shift from �0.12 �C to

�19.92 �C corresponding to addition of 20 wt% and 50 wt% of

BmImCl as portrayed in Fig. 4. The plasticizing effect becomes

more apparent with increasing mass fraction of ionic liquid.

Therefore, the flexibility of polymer chain in CL 5 is higher

than in CL 2. The ionic hopping mechanism is more favorable

in CL 5 which leads to higher ionic conductivity. The result is

in good agreement with previous study. Nevertheless,

different phenomenon is observed for CL 6. Tg is enhanced to

�9.37 �C by adding 10 wt% of BmImCl further. The increase in

Tg (instead of decrease) might be due to the formation of ion

pairs and ion aggregates. Ion pairing and ion aggregation

could obstruct the orientation of the molecules and hence

constrain the flexibility of polymer chains. Therefore, the

decoupling and migration of cations are hindered by the ion

pairs and ion agglomerates.

3.3. Linear sweep voltammetry (LSV)

The maximum operational potential for charge or energy

storage in EDLCs is checked by carrying out LSV study [11].

Fig. 5(a) and (b) describe LSV responds of the EDLC cell with CL

0 and CL 5 polymer electrolytes, respectively. The CL 0-based

EDLC can be operated up to 3.7 V in the potential range be-

tween�1.9 V and 1.8 V, as shown in Fig. 5(a). Upon inclusion of

50 wt% of BmImCl into the polymer electrolyte, the electro-

chemical stability window is expanded to 4 V, where the

cathodic and anodic potentials are detected at �1.8 V and

2.2 V, respectively. Therefore, it can be concluded that

impregnation of ionic liquid can improve the electrochemical

stability of EDLC. Apart from that, the operational current of

EDLC comprising of CL 5 is higher than that of CL 0. This is

mainly assigned to high ion concentration in ionic liquid. The

activated carbon layer would allow the absorption of a large

extent of ions as ionic liquid provides more ions. Hence, the

ion absorption forms the charge accumulation at the elec-

trolyteeelectrode interface and yields the electrical double

layer. Therefore, more energy would be stored in this vicinity.

More electrons will be left and transported from positive

electrode to negative electrode when the ion absorption is

-50 -30 -10 10 30 50 70 90 110 130 150

Hea

t Flo

w (

W/g

)

Temperature (C)

PVA CL 0 CL 2 CL 5 CL 6

–19.92 C

0.12 C

46.58 C

9.37 C

80.15 C

Fig. 4 e DSC thermograms of pure PVA, CL 0 and ionic

liquidebased polymer electrolytes from L50 �C to 150 �C.

increased. This is responsible for higher operational current in

the EDLC based on CL 5 polymer membrane.

3.4. Cyclic voltammetry (CV)

According to the literature, there are three different tech-

niques to find out the capacity of the electrochemical devices

[24]:

i) From cyclic voltammetry

ii) From impedance spectroscopy

iii) From galvanostatic chargeedischarge performance

All these experiments will be analyzed and discussed in

details in this present approach. In order to verify the capac-

itance of the fabricated EDLC cells, the results obtained from

each experiment are compared. Fig. 6 (a) and (b) exemplify CV

curves of type I and II cells, respectively. Type I supercapacitor

shows leaf-like shape with specific capacitance of 0.02 Fg�1,

whereas type II cell illustrates a voltammogram approaching

ideal box-like shape with specific capacitance value of

28.36 Fg�1 [25]. This result is higher than some literatures.

Pandey and his co-workers fabricated EDLC using ionic liquid-

based poly(ethylene oxide) polymer electrolytes and multi-

walled carbon nanotube electrodes. The result is almost 10

times lower than our current work, which is 2.6e3 Fg�1 [23].

Hybrid capacitors using PVA-sodium poly (acrylate) (PAAS)-

potassium hydroxide (KOH) alkaline polymer electrolytes and

nickel hydroxide (Ni(OH)2)/activated carbon (AC) composite

positive electrodes were also assembled by Sun et al. [27].

-10

-8

Cell potential, ΔE (V)

-80

-60

-40

-20

0

20

40

60

80

-4 -3 -2 -1 0 1 2 3 4Cur

ent,

I (

µA)

Cell potential, ΔE (V)

2.2 V–1.8V

(b)

Fig. 5 e (a): LSV response of type I EDLC. (b): LSV response

of type II EDLC.

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8

(a)

(b)

1

Cur

rent

, I (

µA)

Cell potential, ΔE (V)

-6

-4

-2

0

2

4

6

8

10

12

0 0.2 0.4 0.6 0.8 1

Cur

rent

, I (

mA

)

Cell potential, ΔE (V)

Fig. 6 e (a): Cyclic voltammograms of type I EDLC. (b): Cyclic

voltammograms of type II EDLC.

i n t e r n 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 3 9 ( 2 0 1 4 ) 2 9 5 3e2 9 6 3 2959

Higher specific capacitance of 29.8 Fg�1 was obtained for these

hybrid capacitors. This result is expected because of the high

faradic specific pseudocapacitance of Ni(OH)2 [27].

Type I cell is deviated from the ideal rectangular shape

due to the poor contact between electrode and electrolyte.

So, the ions are very difficult to be absorbed onto the elec-

trode surface with this low interfacial contact. The ill-

defined rectangular shape in Fig. 6(a) is also because of the

low conductivity of polymer electrolyte [10]. However, when

ionic liquid is added into the polymer matrix, sticky sample

is produced due to the plasticizing effect of ionic liquid.

Hence, the interfacial contact is improved with this sticky

behavior leading to increase in energy storage in EDLC. The

improvement of interfacial contact could then induce

higher capacitive performance of EDLC. In addition, the

ionic conductivity is enhanced upon addition of ionic liquid

which helps in improving the charge storage capacitive

behavior. The capacitance of type I supercapacitor is

extremely low compared with type II cell. The specific

capacitance value is found to be increased about 131833%

with addition of ionic liquid into the polymer matrix. Apart

from better interfacial contact, the increment of specific

capacitance is attributed to the main feature of ionic liquid.

As aforementioned in introduction and section 3.3, ionic

liquid is comprised solely of ions. Consequently, higher ion

concentration favors the ion migration within the polymer

electrolyte (known as separator in EDLC) and promotes the

charge accumulation at the electrolyteeelectrode boundary.

Eventually, the formation of this electrical double layer in-

creases the capability of energy storage in EDLC. No faradaic

process is detected in the figure entailing no redox reaction

in the EDLC. In addition, higher operational current also

observed in this study. Based on the findings, inclusion of

ionic liquid is a successful way to improve the capacitive

feature of EDLC.

3.5. Electrochemical impedance spectroscopy (EIS)

EIS is a versatile method to assess the electrical behavior of

the assembled cells. Fig. 7(a) and (b) show the complex

impedance spectra of different capacitor cells at ambient

temperature. Both impedance responses illustrate a spike at

low frequency range which is associated with a semicircle at

high frequency region [26]. The semicircular feature is not so

clear in Fig. 7(a), so the scale of the plot is enlarged and shown

in inset of the figure. The linearly rising pattern (or spike) in

the impedance plot reveals the ion absorption at the electro-

deeelectrolyte boundary and hence indicates the capacitive

behavior of EDLC. From both impedance plots, it can be seen

that there are two resistances appearing in the whole fre-

quency regime. Both resistances are determined from the in-

tercepts on x-axis of the complex impedance plot of EDLCs.

The resistance at low frequency range is known as the com-

bination of charge transfer resistance (Rct) and bulk resistance

(Rb), whereas the bulk resistance (Rb) is located at high fre-

quency end, as shown in both figures. The high frequency

intercept (bulk resistance) reflects the bulk properties of

polymer electrolyte [11,23]. Type I cell displays high bulk

resistance that is around 3400 U, however, this resistance is

significantly reduced to 5 U with doping of ionic liquid into

polymer electrolyte for type II supercapacitor. Strong plasti-

cizing effect of ionic liquid is an important contributor in

decreasing the bulk resistance of polymer membrane as dis-

cussed in section 3.1. The ionic mobility and transportation

could be enhanced with this low bulk resistance, resulting in

higher ionic conductivity. This result is in good agreement

with the previous findings in section 3.1. On the contrary, the

charge transfer resistance is one of the bulk behaviors of

electrodeeelectrolyte interface. It is defined as a resistance

that the charge carriers required to overcome during the

charge transfer process from the electrolyte to surface of

electrode. It is noteworthy that the charge transfer resistance

of type II EDLC is radically improved from 1100 U to 4 U upon

addition of ionic liquid into the polymer electrolyte. This

observation indicates the effective role of ionic liquid in

improving the interfacial contact between the electrode and

electrolyte. The specific capacitance of the cells is then eval-

uated using equation (2). As expected, the specific capacitance

of type II cell is higher than type I cell. The specific capaci-

tances of 0.03 Fg�1 and 27.76 Fg�1 are obtained for type I and

type II EDLCs, respectively. The results are comparable with

the CV findings. Again, the abrupt increase in specific capac-

itance of type II EDLC is suggestive of the high ion concen-

tration and good adhesion of the electrolyte to electrode, as

explained in previous study. Superior electrochemical prop-

erties in conjunction with excellent interfacial contact are

Fig. 7 e (a): Nyquist impedance plot of type I EDLC at room temperature from 10 mHz to 100 kHz with closeeup view of the

plot in high frequency region (inset). (b): Nyquist impedance plot of type II EDLC at room temperature from 10 mHz to

100 kHz.

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 3 9 ( 2 0 1 4 ) 2 9 5 3e2 9 6 32960

0

0.2

0.4

0.6

0.8

1

1.2

0 500 1000 1500 2000 2500 3000 3500

Cel

l pot

enti

al, E

(V

)

Time (s)

0

0.2

0.4

0.6

0.8

1

0 500 1000 1500 2000 2500 3000

Cel

lpot

enti

al, E

(V)

Time (s)

Fig. 8 e Galvanostatic chargeedischarge performances of

type II cell over first 5 cycles.

i n t e r n 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 3 9 ( 2 0 1 4 ) 2 9 5 3e2 9 6 3 2961

observed in EDLC comprising of ionic liquid-based polymer

electrolyte.

3.6. Galvanostatic chargeedischarge performance

Galvanostatic chargeedischarge experiment can also be done

to inspect the electrochemical property of EDLC upon the

charge and discharge processes and execute the cyclic dura-

bility test. The GCD result of type II supercapacitor is shown in

Fig. 8. The symmetrical pattern of the charge and discharge

behavior in Fig. 8 implies the superior capacitive nature of

EDLC [12]. As can be clearly seen in Fig. 8, the potential range

of inclined part for charging is between 0.2 V and 1 V. The cell

starts to be charged at 0.2 V instead of 0 V because of the in-

ternal resistance of the supercapacitor. Beyond charging

process, it is followed with a cell discharge curve. The small

drop in the initial part of discharge curve is assigned to the

internal resistance of electrochemical capacitors. This ohmic

loss arises from resistances between electrode and electrolyte,

such as charge transfer resistance and bulk resistance of

Fig. 9 e Schematic diagrams of charging and

polymer electrolyte [11,13]. According to Pandey et al., the

ohmic loss is also attributed to the depletion of polymer

electrolyte [26]. The ion migration and accumulation in the

electrical double layer could reduce the amount of mobile

charge carriers in the polymer membrane causing the deple-

tion. The depletion of polymer electrolytes could increase the

cell resistance. This phenomenon is supported by Fig. 8. The

internal resistance of the cell for both charging and dis-

charging curves increase slightly with increasing cycle num-

ber. When the cycle number is increased, some ions are more

preferred to be paired up rather than be transported into the

electrolyteeelectrode interface as a result of rapid charge

accumulation in the region. This causes the depletion of

polymer electrolyte and thereby rises up the internal

resistance.

The principle of charging and discharging processes

involving ion absorption and desorption is further described

in Fig. 9. When an external electric field is applied, the elec-

trons would be given out and be transferred from positive

electrode to negative electrode. The poles that the electrons

leave from the activated carbon are considered as positively-

charged. The anions from the polymer electrolyte (i.e. ace-

tate anion, CH3COO� and chloride, Cl� in this current project)

could be drawn to this positively charged surface at the pos-

itive electrode through ion absorption process when the

voltage is applied across the cell. On the other hand, the

hydrogen cations (Hþ) in the polymer electrolyte are attracted

to the negative electrode simultaneously. We suggest that 1-

butyl-3-methylimidazolium cation (BmImþ) is very hard to

be attached to the electrode due to its bulky sizewith low ionic

mobility. Therefore, only Hþ is available for the charge accu-

mulation at the surface of negative electrode. The cations and

anions are thus distributed relative to each other over the

distance of the polymer matrix. Eventually, upon the

completion of charging electricity process, the accumulation

of the charge carriers at both electrodes will lead to the for-

mation of electrical double layer and thereby create the en-

ergy storage in EDLC. Conversely, the ions will move away in

the discharging process.

The cycle life test is a vital characterization to determine

the stability of EDLC in terms of electrochemical perfor-

mances after charging and discharging processes. Fig. 10

represents the specific discharge capacitance and Coulombic

discharging processes for a typical EDLC.

0

20

40

60

80

100

120

0

5

10

15

20

25

30

35

0 100 200 300 400 500

Cou

lom

bic

effi

cien

cy (

%)

Spec

ific

dis

char

ge c

apac

itan

ce, C

sp(F

g- 1)

Cycles

Fig. 10 e Specific capacitance and Coulombic efficiency of

type II EDLC over 500 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 3 9 ( 2 0 1 4 ) 2 9 5 3e2 9 6 32962

efficiency of supercapacitor over 500 cycles. The energy and

power densities are thus calculated using equations 4 and 5,

respectively. The result obtained over 500 cycles is then

plotted in Fig. 11. The initial specific discharge capacitance of

31.28 Fg�1 with Coulombic efficiency of 90% is observed for

type II cell in Fig. 10. This capacitance value is similar with the

result obtained in CV and impedance spectroscopy. However,

the capacity of cell decreases with cycle number. The energy

and power densities also possess the same pattern. The de-

creases in the electrochemical performance might be due to

the depletion of polymer electrolyte. The amount of charge

carriers within the polymer membrane is reduced with

increasing cycle number. The internal resistance of cell is

increasedwith cycle number as explained as aforesaid. So, the

ions requires higher amount of energy to be transported to-

wards the surface of electrodewith higher internal resistance.

These ions have to overcome this barrier in order to form the

electrical double layer in the electrodeeelectrolyte region. As a

result, the capacitive behavior is declined. The specific

discharge capacitance is reduced to 20.98 Fg�1 with 98% of

efficiency upon charge and discharge for 500 cycles. On the

other hand, the initial energy density is 2.39Wh kg�1; however

it has been reduced 1.45 Wh kg�1 in 500th cycles. A small

change about 11. 67% in power density, from 19.79 W kg�1 to

17.43 Wh kg�1, is also observed in Fig. 11. From both figures,

we also observe that the specific capacitance, energy density

17

17.5

18

18.5

19

19.5

20

0

0.5

1

1.5

2

2.5

3

0 100 200 300 400 500

Pow

er d

ensi

ty, P

(W

kg-1

)

Ene

rgy

dens

ity,

E (

Whk

g-1)

Cycles

Fig. 11 e Energy density and power density of type II EDLC

over 500 cycles.

and power density become almost constant above 400th cy-

cles. This signifies that the cell can remain in its electro-

chemical stability without any abrupt changes in

electrochemical performances upon charge and discharge for

400 cycles, suggesting that the ionic liquid-based polymer

electrolyte is a best choice as separator in supercapacitor. The

most conducting ionic liquid-based polymer electrolyte is also

a promising candidate by maintaining its Coulombic effi-

ciency above 90% in 500 cycles of charging and discharging.

According to the findings, CL 5 is very suitable to be applied in

EDLC application as the electrochemical performance has

been greatly improved.

4. Conclusion

PVA-based polymer electrolytes were prepared by solution

casting method. The ionic conductivity increased with tem-

perature due to the polymer expansion effect. Polymer elec-

trolyte containing 50 wt% of BmImCl offered the maximum

ionic conductivity of (7.31 � 0.01) mS cm�1 at 120 �C. The

samples obeyed non-Arrhenius VTF rule as convex upward

curved profiles were shown in temperature dependence-ionic

conductivity studies. This principle stated that the ion trans-

portation mechanism is coupled with highly flexible polymer

chain which promotes the ion hopping process. Upon

impregnation of BmImCl, the Tg of polymer electrolytes is

lowered down to sub-ambient value. The decrease in Tg hel-

ped in softening the polymer backbone. EDLC containing the

most conducting polymer electrolyte was assembled. The

fabricated EDLC can be charged up to 4.8 V, as illustrated in

LSV study. Doping of ionic liquid into polymer electrolyte also

increased the capacitive nature of EDLC. The specific capaci-

tance of 28.36 Fg�1 was achieved with better electrochemical

characteristic in cyclic voltammogram. The specific capaci-

tance value obtained from impedance spectra and galvano-

static chargeedischarge test are also comparable with the

result from CV study. The inclusion of ionic liquid not only

improves the interfacial contact between electrode and elec-

trolyte, but also increases the electrochemical property of

supercapacitors. Doping of ionic liquid into PVA-based poly-

mer electrolyte is a good prospect for improving the electro-

chemical performance of an energy storage device.

Acknowledgment

This work was supported by the High Impact Research Grant

(J-21002-73851) from University of Malaya. Gratitude also goes

to “Peruntukan Penyelidikan Pascasiswazah (PPP), Universiti

Malaysia”. One of the authors, Chiam-Wen Liew gratefully

acknowledges the “Skim Bright Sparks Universiti Malaya”

(SBSUM) for scholarship awarded.

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