characterization of ionic liquid added poly(vinyl alcohol ...repository.um.edu.my/100256/1/wendy...

ww.sciencedirect.com

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 2

Available online at w

ScienceDirect

journal homepage: www.elsevier .com/locate/he

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

[email protected] (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.


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.


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.


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.


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


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.


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.


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.


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.

r e f e r e n c e s

[1] Frackowiak E. Carbon materials for supercapacitorapplication. Phys Chem Chem Phys 2007;9:1774e85.

[2] Lei C, Wilson P, Lekakou C. Effect of poly(3,4-ethylenedioxythiophene) (PEDOT) in carbon-basedcomposite electrodes for electrochemical supercapacitors. JPower Sources 2011;196:7823e7.


[3] Kumar Y, Pandey GP, Hashmi SA. Gel polymer electrolytebased electrical double layer capacitors: comparative studywith multiwalled carbon nanotubes and activated carbonelectrodes. J Phys Chem C 2012;116:26118e27.

[4] An KH, Jeon KK, Heo JK, Lim SC, Bae DJ, Leez YH. High-capacitance supercapacitor using a nanocomposite electrodeof single-walled carbon nanotube and polypyrrole. JElectrochem Soc 2002;149:1058e62.

[5] Peng C, Zhang S, Jewell D, Chen GZ. Carbon nanotube andconducting polymer composites for supercapacitors. ProgNat Sci 2008;18:777e88.

[6] Portet C, Taberna PL, Simon P, Flahaut E. Influence of carbonnanotubes addition on carbonecarbon supercapacitorperformances in organic electrolyte. J Power Sources2005;139:371e8.

[7] Wang G-X, Zhang B-L, Yu Z-L, Qu M-Z. Manganese oxide/MWNTs composite electrodes for supercapacitors. SolidState Ionics 2005;176:1169e74.

[8] Gray FM. Solid polymer electrolytes: fundamentals oftechnological applications. United Kingdom: WileyeVCH;1991.

[9] Rajendran S, Sivakumar M, Subadevi R. Investigations on theeffect of the various plasticizers in PVAePMMA solidpolymer blend electrolytes. Mater Lett 2004;58:641e9.

[10] Armand M, Endres F, MacFarlane DR, Ohno H, Scrosati B.Ionic liquid materials for the electrochemical challenges ofthe future. Nat Mat 2009;8:621e9.

[11] Fisher AS, Khalid MB, Widstrom M, Kofinas P. Solid polymerelectrolytes with sulfur based ionic liquid for lithiumbatteries. J Power Sources 2011;196:9767e73.

[12] Singh PK, Kim K-W, Rhee H-W. Ionic liquid (1-methyl 3-propyl imidazolium iodide) with polymer electrolyte forDSSC application. Polym Eng Sci 2009;49:862e5.

[13] Hirankumar G, Selvasekarapandian S, Kuwata N,Kawamura J, Hattori T. Thermal, electrical and opticalstudies on the poly(vinyl alcohol) based polymer electrolytes.J Power Sources 2005;144:262e7.

[14] Selvasekarapandian S, Hirankumar G, Kawamura J,Kuwata N, Hattori T. 1H solid state NMR studies on theproton conducting polymer electrolytes. Mater Lett2005;59:2741e5.

[15] Ramesh S, Liew C-W, Ramesh K. Evaluation andinvestigation on the effect of ionic liquid onto PMMAePVCgel polymer blend electrolytes. J Non-cryst. Solids2011;357:2132e8.

[16] Arof AK, Kufian MZ, Syukur MF, Aziz MF, Abdelrahman AE,Majid SR. Electrical double layer capacitor using poly(methylmethacrylate)-C4BO8Li gel polymer electrolyte andcarbonaceous material from shells of Mata Kucing(Dimocarpus longan) fruit. Electrochimi Acta 2012;74:39e45.

[17] Amitha FE, Reddy ALM, Ramaprabhu S. A non-aqueouselectrolyte-based asymmetric supercapacitor with polymerand metal oxide/multiwalled carbon nanotube electrodes. JNanopart Res 2009;11:725e9.

[18] Mitra S, Shukla AK, Sampath S. Electrochemical capacitorswith plasticized gel-polymer electrolytes. J Power Sources2001;101:213e9.

[19] Yu H, Wu J, Fan L, Lin Y, Xu K, Tang Z, et al. A novel redox-mediated gel polymer electrolyte for high-performancesupercapacitor. J Power Sources 2012;198:402e7.

[20] Liew C-W, Ramesh S, Arof AK. A novel approach on ionicliquid-based poly(vinyl alcohol) proton conductive polymerelectrolytes for fuel cell applications. Int J Hydrogen Energy2014;39:2917e28.

[21] Liew C-W, Ramesh S, Arof AK. Good prospect of ionic liquidbased-poly(vinyl alcohol) proton conducting polymerelectrolytes for supercapacitors with excellent electrical,electrochemical and thermal Properties. Int J HydrogenEnergy 2014;39:2953e63.

[22] Lee KY, Kim CS, Kim H, Cheong M, Mukherjee DK, Jung K-D.Effects of halide anions to absorb SO2 in ionic liquids. BullKorean Chem Soc 2010;31:1937e40.

[23] Pandey GP, Kumar Y, Hashmi SA. Ionic liquid incorporatedPEO based polymer electrolyte for electrical double layercapacitors: a comparative study with lithium andmagnesium systems. Solid State Ionics 2011;190:93e8.

[24] Sun Z, Yuan A. Electrochemical performance of nickelhydroxide/activated carbon supercapacitors using amodified polyvinyl alcohol based alkaline polymerelectrolyte. Chin J Chem Eng 2009;17:150e5.

[25] Lei C, Markoulidis F, Ashitaka Z, Lekakou C. Reduction ofporous carbon/Al contact resistance for an electric double-layer capacitor (EDLC). Electrochim Acta 2013;92:183e7.

[26] Fang B, Binder L. A novel carbon electrode material for highlyimproved EDLC performance. J Phys Chem B2006;110:7877e82.

[27] Zheng JP. Resistance distribution in electrochemicalcapacitors with a bipolar structure. J Power Sources2004;137:158e62.

characterization of ionic liquid added poly(vinyl alcohol ...repository.um.edu.my/100256/1/wendy...

Documents