a novel approach on ionic liquid-based poly(vinyl

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A novel approach on ionic liquid-based poly(vinylalcohol) proton conductive polymer electrolytes forfuel cell applications

Chiam-Wen Liew, S. Ramesh*, A.K. Arof

Centre for Ionics of 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 8 December 2012

Received in revised form

4 July 2013

Accepted 23 July 2013

Available online xxx

Keywords:

PVA

Ionic liquid

Proton conductivity

Fuel cell

Power density

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

Please cite this article in press as: Liew C-polymer electrolytes for fuel cell applicj.ijhydene.2013.07.092

0360-3199/$ e see front matter Copyright ªhttp://dx.doi.org/10.1016/j.ijhydene.2013.07.0

a b s t r a c t

The preparation of proton conducting-polymer electrolytes based on poly(vinyl alcohol)

(PVA)/ammonium acetate (CH3COONH4)/1-butyl-3-methylimidazolium chloride (BmImCl)

was done by solution casting technique. The ionic conductivity increased with ionic liquid

mass loadings. The highest ionic conductivity of (5.74 � 0.01) mS cm�1 was achieved upon

addition of 50 wt% of BmImCl. The thermal characteristic of proton conducting-polymer

electrolytes is enhanced with doping of ionic liquid by showing higher initial decomposi-

tion temperature. The most conducting polymer electrolyte is stable up to 250 �C. Atten-

uated total reflectance-Fourier Transform Infrared (ATR-FTIR) confirmed the complexation

between PVA, CH3COONH4 and BmImCl. Polymer electrolyte membrane fuel cell (PEMFC)

was fabricated. This electrochemical cell achieved the maximum power density of

18 mW cm�2 at room temperature.

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

reserved.

1. Introduction component in PEMFCs is the proton exchange membrane

Depletion of non-renewable fossil fuel catalyzes the devel-

opment of environmentally friendly alternative energy sour-

ces. Fuel cells are promising candidates as power sources for a

wide range of applications, ranging from portable devices to

electric vehicles [1,2]. Fuel cells are of great interest in the

recent research due to their attractive properties. They offer

many advantages, such as environmentally benign, high en-

ergy efficiency and lower emission of pollutants [3,4]. Fuel

cells also do not require the need for rapid recharging process

in the presence of electricity compared to the lithium

rechargeable batteries [5]. Apart from that, fuel cells exhibit

higher energy density than the batteries [6]. The main

1; fax: þ60 3 7967 4146.il.com (C.-W. Liew), rame

W, et al., A novel approaations, International Jo

2013, Hydrogen Energy P92

(PEM) which is used as proton (or charge carriers) provider

from anode to cathode. The basic requirement of PEM is the

membrane must be proton conductive with some satisfactory

properties like excellent mechanical strength and chemically

stable. Perfluorosulfonated membrane, Nafion produced by

Dupont has widely been used as host polymer in PEMFCs.

However, high methanol transfer is the main shortcoming of

using Nafion as primary polymer membrane in fuel cells [6,7].

High methanol crossover may cause the loss of fuel, slow

methanol oxidation kinetics and formation of carbon mon-

oxide (CO) intermediate on the platinum catalyst layer at the

cathode as well as cell polarization. As a consequence, the

performance of this electrochemical cell could be decreased

[email protected] (S. Ramesh), [email protected] (A.K. Arof).

ch on ionic liquid-based poly(vinyl alcohol) proton conductiveurnal of Hydrogen Energy (2013), http://dx.doi.org/10.1016/

ublications, LLC. Published by Elsevier Ltd. All rights reserved.

mailto:[email protected]



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http://dx.doi.org/10.1016/j.ijhydene.2013.07.092



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 x x x ( 2 0 1 3 ) 1e1 22

[6e8]. It also possesses some drawbacks such as expensive,

high water swelling characteristic which reduces the lifespan

of the membrane and decrease in conductivity at elevated

temperature [9]. So, the utilization of Nafionmembrane in fuel

cell is limited. Therefore, many attempts have been done to

overcome the weakness of Nafion.

Non-perfluorosulfonated polymer membrane was

employed to replace Nafion in DMFC. Poly(vinyl alcohol) (PVA)

is a potential material because of its cheaper price [2]. PVA is a

biodegradable semi-crystalline synthetic polymer bearing

with hydroxyl functional group [7,10,11]. Superior chemical

stability, non- toxic, high hydrophilicity, ease of preparation

and biocompatibility are advantages of PVA [10e13]. Further-

more, PVA exhibits excellent thermal and chemical stabilities

and high abrasion resistance as well as high flexibility [13,14].

PVA also possesses high dielectric permittivity and excellent

capacity for charge storagewhich is an important feature for a

supercapacitor, as reported by Hirankumar et al. [15]. In order

to prepare proton-conducting polymer electrolytes, ammo-

nium salt must be embedded to provide the charge carriers as

it is well-known as proton donor. Ammonium acetate

(CH3COONH4) was chosen in this project due to its plasticizing

effect. However, the ionic conductivity is relatively low.

Therefore, much effort has been devoted to enhance the ionic

conductivity, for instance polymer blending, addition of

plasticizer, incorporation of filler, mixed salt system and

doping of ionic liquid. Among all these approaches, impreg-

nation of ionic liquid can be considered as the most feasible

method because it is well known as environmentally friendly

compound. Ionic liquid brings multiple advantages such as

wider electrochemical stability, relatively high conductivity,

negligible vapor pressure, high resistance to chemical sub-

stances and excellent thermo-stability, as reported in our

previous works [16e18].

Several PVA-based fuel cells had been fabricated and

analyzed by researchers. Higa and his co-workers fabricated a

direct methanol fuel cell (DMFC) with a polymer electrolyte

membrane where PVA is cross-linked 2 mol% of 2-methyl-1-

propanesulfonic acid (AMPS). Themaximumpower density of

2.4mWcm�2 was delivered by this DMFC [7]. In addition, PVA-

based nanocomposite polymer electrolytes were frequently

used as separator in direct methanol alkaline fuel cell

(DMAFC) and studied by Yang CC research group. The

maximum power density of 15.3 mW cm�2 was achieved

using PVA/potassium hydroxide (KOH) with doping of 20% of

fumed silica [6]. However, lower maximum power density of

7.54mW cm�2 was obtained in PVA/titania (TiO2)-based DMFC

[4]. Sekhon et al. synthesized polymer electrolyte containing

poly(vinylidene fluoride-co-hexafluoropropylene (PVdF-co-

HFP), triflic acid (HCF3SO3) and 2,3-dimethyl-1-octylimidazo-

lium triflate (DmOImTf) for a PEMFC. Power density of

0.5 mW cm�2 was obtained. This result is quite low compared

to the Nafion membrane. However, it is comparable with the

literature [19]. The objective of the current project is to prepare

environmentally friendly polymer electrolytes with high

electrical properties and excellent thermal stability. Polymer

electrolyte membrane fuel cells (PEMFCs) are thus fabricated

using prepared polymer films. The electrochemical perfor-

mances of PEMFCs such as power density are eventually

studied in the present work. Another aim of this current

Please cite this article in press as: Liew C-W, et al., A novel approapolymer electrolytes for fuel cell applications, International Joj.ijhydene.2013.07.092

project is to explore the effect of ionic liquid onto the elec-

trochemical properties of polymer electrolytes and fuel cells.

2. Experimental

2.1. Materials

Proton conducting polymer electrolytes comprising of PVA,

CH3COONH4 and BmImCl were prepared in this present work.

PVA (Sigma-Aldrich, USA, 99% hydrolyzed with molecular

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

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

ionic liquid, respectively. All the materials were used as

received.

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

Ionic liquid based-poly(vinyl alcohol) proton conducting

polymer electrolytes were prepared by solution casting tech-

nique. PVA was initially dissolved in distilled water. Appro-

priate amount of CH3COONH4was subsequentlymixed in PVA

solution. The weight ratio of PVA:CH3COONH4 was kept at

70:30. Different mass fraction of BmImCl was thus embedded

into the PVA-CH3COONH4 mixture to prepare ionic liquid-

based proton conducting polymer electrolyte. CL 2, CL 5 and

CL 6 were the designations of polymer electrolytes with

respect to addition of 20 wt%, 50 wt% and 60 wt% of BmImCl,

whereas CL 0 was for the polymer electrolyte without incor-

poration of BmImCl. The resulting solution was stirred thor-

oughly and heated at 70 �C for few hours until a homogenous

colorless solution is obtained. The solution was eventually

casted on glass Petri dish and dried in an oven at 60 �C to

obtain a free-standing proton conducting polymer electrolyte

film.

2.3. Characterization of ionic liquid based-PVA protonconducting polymer electrolytes

2.3.1. Ambient temperature-ionic conductivity studiesFreshly prepared samples were subjected to ac-impedance

spectroscopy for ionic conductivity determination. The

thickness of the film was measured by digital micrometer

screw gauge. The ionic conductivity of the polymer electro-

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

ance analyzer over a frequency regime between 50 Hz and

5 MHz at ambient temperature. The data acquisition was

recorded at a signal level of 10mV. Proton conducting polymer

electrolytes were sandwiched on the sample holder under

spring pressure in the configuration of stainless steel (SS)

blocking electrode/polymer electrolyte/SS electrode.

2.3.2. Thermogravimetric analysis (TGA)TGA was accomplished by thermogravimetric analyzer, TA

Instrument Universal Analyzer 2000 with Universal V4.7A

software. Sample weighing 2e3 mg placed into 150 ml of silica

crucible. The sampleswere then heated from 25 �C to 600 �C at

a heating rate of 50 �C min�1 in a nitrogen atmosphere with a

flow rate of 60 ml min�1.




50

100

150

200

250

300

350

400

Rs+Rb

Rs

i n t e r n a t i o n a l j o u r n a l o f h yd r o g e n e n e r g y x x x ( 2 0 1 3 ) 1e1 2 3

2.3.3. Attenuated total reflectance-Fourier TransformInfrared (ATR-FTIR)Themoscientific Nicolet iS10 FTIR Spectrometer (from USA)

was employed to perform ATR-FTIR study. This spectrometer

is equipped with an ATR internal reflection system. The FTIR

spectra were recorded with a resolution of 4 cm�1 in trans-

mittance mode over the wavenumber range from 4000 to

650 cm�1. The FTIR spectra and peak deconvolution were

scrutinized by OMNIC 8 softwarewhich is provided by Thermo

Fischer Scientific Inc. The transmittance mode of FTIR spectra

was initially converted into absorbance mode for peak

deconvolution process. In order to deconvolute the FTIR

spectra, baseline correction and curve fitting must be imple-

mented. The FTIR curve was fitted with gaussian-lorentzian

mixed mode.

2.4. Fuel cell fabrication

The most conducting polymer electrolyte was used for PEMFC

fabrication. The polymer membrane was soaked in distilled

water for 5 min prior the fabrication. The surface area of

electrodes is 16 cm2. The polymer electrolyte was then placed

in the PEMFC kit which is purchased from H-tec, Germany.

The assembled kit was eventually subjected to the Electro-

lyzer 10 (from H-tec, Germany) for electrochemical analysis

with hydrogen flow rate of 10 cm3 min�1 and oxygen flow rate

of 5 cm3 min�1.

2.4.1. Cell performancesThe performance of PEMFC was done by fuel cell measure-

ment setup which is comprised of Programmable DC power

supply (Model no: 3644A, Array Electronics Co. Ltd. from

Taiwan), rheostat (Model no: ZX97E, Tinsley from United

Kingdom) and Digital Bench Type True RMS multimeter

(UT803, Uni-Trend from Guangdong, Zhina). DC power supply

is employed to provide the current flow through the cell,

whereas the rheostat is used to adjust the current flow. The

measurements of voltage and current flow were probed using

multimeter. The cell potential in every 10 mA of current was

noted down, starting from 10 mA. The current density was

determined by dividing the operational current into surface

area of electrodes (16 cm�2). On the other hand, the power

densitywas calculatedusing the fundamental equationbelow:

P ¼ IV (1)

where P is power (W), I is operational current (A) and V is cell

potential (V). Again, the power density was eventually

computed by dividing the power into surface area of

electrodes.

00 50 100 150 200 250 300 350 400

Fig. 1 e ColeeCole impedance plot of CL 0 at room

temperature.

3. Results and discussion

3.1. Ambient temperature-ionic conductivity studies

The ionic conductivity of polymer electrolytes is determined

using the following equation:

s ¼ lRbA

(2)


where l is the thickness of polymer electrolytes (cm); Rb is bulk

resistance (U) of polymer electrolytes and A is the known

surface area (cm2) of blocking electrodes. Fig. 1 shows a typical

impedance plot of CL 0 at ambient temperature. The Rb is

determined as shown in Fig. 1. Since the impedance plot is not

starting from origin, so the series resistance (Rs) is appeared in

the figure. The total resistance (combination of Rb and Rs) is

calculated from the intercept of the spike with the extrapo-

lation of the semicircular region on Z real axis. The Rb is

computed from the deduction of the total resistance with the

initial Rs. On the other hand, the spike of the plot indicates the

charge accumulation at the electrolyteeelectrode interface.

Fig. 2 depicts the ionic conductivity of proton-conducting

polymer electrolytes at different mass fraction of BmImCl.

Upon inclusion of ionic liquid, the ionic conductivity is

increased. As expected, the ionic conductivity increases with

mass loadings of ionic liquid, up to an optimum level. The

ionic conductivity increases gradually when 10 wt% of

BmImCl is added into the polymer electrolyte. However, the

ionic conductivity is found to be increased rapidlywith doping

of 20 wt% of BmImCl. The highest ionic conductivity of

(5.74 � 0.01) mS cm�1 corresponding to addition of 50 wt% of

BmImCl is observed. This conductivity has been increased by

two orders of magnitude in comparison to the polymer elec-

trolyte without impregnation of ionic liquid. The abrupt in-

crease in ionic conductivity is due to strong plasticizing effect

of ionic liquid [16,18]. Strong plasticizing effect of ionic liquid

aids to soften the polymer backbone and hence increases the

flexibility of polymer chain. The proton can be transported

easily within the polymer matrix with highly flexible polymer




-5.50

-5.00

-4.50

-4.00

-3.50

-3.00

-2.50

-2.00

0 10 20 30 40 50 60 70 80 90Weight percent of BmImCl (wt %)

Fig. 2 e The ionic conductivity of proton conducting

polymer electrolytes at different mass ratio of BmImCl.

NN+

H

NN H+

Deprotonation+

..

CH2 CH CH2 CH

O OH

n

H

CH2 CH CH CH

O OH OH

n

H

NC

+

N

NN..


chains. Besides, higher flexibility of polymer chains improves

the mobility of polymer segments and assists the ionic

transportation within polymer complexes. Consequently, the

ionic conductivity is higher compared to the polymer elec-

trolyte without doping of ionic liquid. Moreover, ionic liquid

increases the amorphousness of the polymer matrix. Ionic

liquid could weaken the transient coordinative bonds among

the molecules in the crystalline region and thus turn the

polymer chains into flexible characteristic leading to higher

amorphous degree of polymer complexes. Ionic liquid also

promotes the ionic conduction process by providing more

conducting sites. This can be done by interacting BmImþ with

hydrogen of the side chain in the polymer backbone. This

interaction not only provides more conducting sites, but it

also provides more mobile proton upon the deprotonation.

In order to understand the principle in the proton-con-

ducting polymer electrolytes, the proton hopping mechanism

must be well studied. We propose a mechanism pertaining to

transport of conducting ions for proton-conducting polymer

electrolytes illustrated as follows. The ionic liquid: imidazo-

lium cation (BmImþ) and chloride (Cl�) can be easily detached

from the transient partial bonding due to the bulky size of the

cation. Thus, the hydrogen at C2-position of the mobile

BmImþ is initially de-protonated to form a stabilized carbene

[21,22].

CH2 CH CH2 CH

O OH

n

H

NC

+

N

Cl OCH3CO

CH2

or / and

3

3-butyl-1-methyl-4,5-di


This carbene could then interact with the hydrogen in

hydroxyl group of PVA and result carbocation in the imida-

zolium ring.

This interaction leads to the partial hydrogen bonding

between oxygen and hydrogen. In addition, the de-proton-

ated hydrogen in this step can help in accelerating the proton

conduction as it is in mobile state. When the hydrogen

bonding is partially bonded, the dissociated chloride and

acetate anions from ionic liquid and salt will prefer to

interact with the carbocation and thus break the hydrogen

bond between the imidazolium cations and side chain of

PVA. After the disintegration of hydrogen bonding, the oxy-

gen atom in the hydroxyl group of PVA turns into negatively

charged or forms anions. Therefore, these electron deficient

oxygen anions would accept the electron from proton from

the loosely bounded ammonium cations or from imidazo-

lium cations. Two possible side products could be formed in

this segment, viz. 3-butyl-2-chloro-1-methyl-4,5-dihydro-1H-

imidazole and 3-butyl-1-methyl-4,5-dihydro-1H-imidazole-2-

carboxylic acid methyl ester, as shown in the mechanism

below.

CH CH2 CH

O OH

n NN

Cl

NN

OOCCH3

: :..

+

or / and

-butyl-2-chloro-1-methyl-4,5-dihydro-1H-imidazole

hydro-1H-imidazole-2-carboxylic acid methyl ester





Beyond this step, there are two possibilities of proton

conduction process. The proton transportation can be gener-

ated from the hydrogen in ammonium cations or the

hydrogen in BmIm cations. The dissociated proton from

loosely bounded ammonium cations of doping salt would

then interact with this negatively charged oxygen and result

the formation of hydroxyl group through hydrogen bonding.

This partial hydrogen bonding would result the CONH coor-

dination bond. On the other hand, the detached chloride or/

and acetate anions will abstract an electron from hydrogen.

This hydrogen will thus leave the hydroxyl group and hence

turn into anions. The adjacent hydrogen could interact with

this oxygen and hence generate the proton transportation

from site to another adjacent site.

CH2

CH CH2

CH

O O

n

H+

Cl OCH3CO

H

CH2

CH CH2

CH

O

n

OH: : ..

or / and

: : ..

Apart from that, we suggest that proton conduction can

also take place at the BmIm cations. For this proton transport,

BmIm cations could interact with the oxygen in PVA and form

NeO bond temporarily. Similarly, the detached chloride or/

and acetate anions will abstract an electron from hydrogen of

another branched site and produce oxygen which is appeared

as anions. As next step, the electron from the hydrogen from

C2-position in BmIm cations could be donated to this electron-

deficient oxygen. In order to form a stable compound, the NeO

bond must be broken down. Hence, this bond detachment

could lead to the formation of carbene, which is an important

chemical compound in weakening the transient coordination

bond within the macromolecules. Eventually, the proton

conduction mechanism is repeated again with the formation

of carbene. These two possible mechanisms will cause

continuous ionic hopping mechanism in the polymer matrix.

CH2

CH CH2

CH

O O

n

Cl OCH3CO

H

NN+

CH2

CH CH2

CHn

OO

NN

H

NN

: : ..

or / and

: :

..

+

..

Beyond adulteration of 50 wt% of BmImCl, the ionic con-

ductivity is found to be lower in value. This is ascribed to the

agglomeration of ions. Excessive ions in the polymer electro-

lyte could form ion pairs and ion aggregates. These ion pairs

and ion aggregates might block the conducting pathway and

hinder the proton from transportation, leading to lower ionic

conductivity. The amount of proton for conduction is also

decreased in the presence of ion pairs and ion aggregates. At

high concentration of ionic liquid, NH4þ cations could rather

form ion pairs with other anions than provide proton for

transportation. Similar observation goes to BmImþ cations.

These cations help in providing the conducting site as afore-

mentioned above. However, at highmass loadings, the cations

could form ion neutral pairs with anions and thus turn the


cations into immobile state. As a consequence, the conducting

sites could be lesser and ultimately reduce the ionic conduc-

tivity. These neutral ion pairs and ion aggregates could

impede the conducting pathway and reduce the flexibility of

polymer chains. This phenomenon can be proven by deter-

mining the activation energy of the proton transportation. The

activation energies of polymer electrolytes containing 20 wt%,

50 wt% and 60 wt% of BmImCl are 7.54, 7.11 and 7.74 meV,

respectively (not shown in this manuscript). Higher activation

energy of the polymer electrolytes infers higher energy that

the proton requires to overcome the barriers for the trans-

portation. Therefore, we can conclude that the lower ionic

conductivity of the polymer electrolytes containing highmass

fraction of BmImCl is attributed to the formation of ion pairs

and ion aggregates as the ion pairs can block the transport

pathway within the polymer chains.

3.2. Thermogravimetric analysis (TGA)

Fig. 3 portrays thermogravimetric analysis for pure PVA, CL

0 and ionic liquid-based polymer electrolytes. Pure PVA shows

three major weight losses, as described in Yang et al. [14].

However, four thermal degradation steps are observed for salt

and/or ionic liquid-doped polymer electrolytes. The initial

thermal degradation in the region of 25 �Ce125 �C is due to the

dehydration of water or moisture as polymer is a hydroscopic

compound [23]. The insignificant weight loss in this stage is

also due to the elimination of impurities in the polymer

electrolytes. Small weight losses of 5%, 5% and 6% are

observed for pure PVA, CL 0 and CL 6, respectively in this re-

gion. However, higher mass losses of 9% have been observed

for CL 2 and CL 5. Beyond this stage, the weight of all the

samples remains the same until 200 �C. Upon the dehydration,

ammonium acetate could be decomposed easily into acet-

amide, CH3C(O)NH2.

On the contrary, two PVA polymer chains might be cross-

linked as a result of water removal. This elimination could

lead to the formation of ether cross-linkages [11]. Above

200 �C, an abrupt decrease in weight is observed for all the

samples. Among all the samples, pristine PVA portrays the

highest mass loss from 210 �C to 235 �C in this segment.

Elimination of side chain in polymer backbone of PVA is the

main attributor to this thermal degradation for pure PVA [14].

Apart from formation of ether cross-linkages, the polymer




0

20

40

60

80

100

120

0 100 200 300 400 500 600

Wei

ght (

%)

Pure PVA CL 0 CL 2 CL 5 CL 6

Fig. 3 e Thermogravimetric analysis of pure PVA, CL 0, CL

2, CL 5 and CL 6.


backbone of PVA could start a rapid chain stripping on the

polymer backbone upon heating and hence turn into conju-

gated double bonds, giving rise to formation of polyene, as

illustrated as below [11]:

Upon addition of salt and ionic liquid, the mass loss in this

stage is significantly decreased. Besides, the starting decom-

position temperature in this stage is improved. These obser-

vations deduct the contribution from the complexation

between salt and/or ionic liquid and PVA. This is because

higher energy would be required to break the transient coor-

dination bond in the complexation process. Weight losses of

32%, 43%, 33% and 44% corresponding to the temperature

range of 240 �Ce245 �C, 245 �Ce285 �C, 250 �Ce305 �C,235 �Ce245 �C are attained for CL 0, CL 2, CL 5 and CL 6,

respectively. The contributor of this thermal decomposition

stage is suggestive of the degradation of acetamide where its

decomposition temperature is around 222 �C. The chain

stripping mechanism is less favorable in CL 0 when the salt is

complexed with PVA. In contrast, the carbene produced from

ionic liquid interacts with the hydrogen from hydroxyl group

in PVA and forms the coordination bond. Therefore, the chain

stripping process might also be absent in the polymer


electrolytes containing ionic liquid. Moreover, BmImCl starts

to be degraded at 237 �C as reported in Lee et al. [25]. As a

result, decomposition of BmImCl also causes the mass loss in

this region. Hence, the weight loss of ionic liquid-based

polymer electrolytes is found to be higher than CL 0 with this

additional attributor. This is in good agreementwith the result

obtained where higher weight loss is obtained for those ionic

liquid-plasticized polymer electrolytes. As can be seen, CL 5

depicts the highest decomposition temperature. This implies

that the most conducting polymer electrolytes exhibit better

thermal properties than other polymer complexes.

A consecutive enormous loss in weight is observed

thereafter for all polymer electrolytes. However, pure PVA

exemplifies insignificant mass loss of 8% from 235 �C until

410 �C. The weight loss of all polymer electrolytes remains in

the regime of 20e30%. CL 0 has weight loss of 20%, starting

from 245 �C until 375 �C, whereas CL 2 exhibits mass loss of

21%, from 285 �C to 365 �C. Similarly, CL 6 also shows around

21% of weight loss in the temperature range of 245 �C and

350 �C. On the contrary, the weight loss of CL 5 is increased

by 5%, which is around 26% whereby the degradation tem-

perature starts from 305 �C to 355 �C. The thermal degra-

dation in this phase arises from the chemical degradation

processes. These chemical decomposition processes include

the breakdown of ether cross-linkages and random chain

scissoring between carbonecarbon bonds [11]. Random bond

scission yields two intermediate products and thus

forms methyl-terminated allylic polyene in cis and trans

isomerisms via elimination process, as demonstrated as

below [24].

Final weight loss is attained at elevated temperature. Pure

PVA and CL 0 show 20% and 26.8% of weight losses, along with

12% and 11% of mass residues at 575 �C, respectively. Ionicliquid-based polymer electrolytes display lower mass loss in

comparison to CL 0. CL 2 exhibits 21% of weight loss from

365 �C to 570 �Cwith residualmass of 3%, whereas CL 5 reveals

higher mass loss, which is around 25% with 6% of residue in

the temperature range of 355 �C-565 �C. CL 6 starts to loss its

weight around 22% with 9% of residue from 350 �C to 575 �C.This final weight loss is connected to the breakdown of the

polymer backbone as reported in Yang et al. [14]. Upon further

heating, the double bond of polyene would be broken down

into single bond and eventually converted into aliphatic

polymer chains at this stage [24]. Beyond this stage, theweight





of samples remains unchanged. This observation infers the

fully decomposition of the polymer matrix. Although the re-

sidual mass of ionic liquid-based polymer electrolytes is

lesser, however, CL 5 is still a promising candidate as polymer

electrolyte due to its higher first decomposition temperature.

3.3. Attenuated total reflectance-Fourier TransformInfrared (ATR-FTIR)

The PVAeCH3COONH4 polymer electrolyte systems have been

prepared and investigated. The FTIR results and the

complexation between PVA and CH3COONH4 have been

elucidated in the literature [15,20,26e32]. Therefore, the

interaction between PVA and CH3COONH4 is not discussed in

this present work. The interactions between ionic liquid and

ammonium acetate-doped polymer electrolyte are also

analyzed and discussed in details. Fig. 4 depicts the ATR-FTIR

spectra of CL 0, BmImCl, CL 2, CL 5 and CL 6. On the other

hand, the band assignment of the spectra is summarized in

Table 1. The ATR-FTIR spectrum of CL 5 is investigated as it

achieves the maximum ionic conducting as aforementioned

in section 3.1. Upon adulteration of BmImCl, sixteen new

peaks have been formed by comparing Fig. 4(a) with Fig. 3(d).

These peaks are CeH vibrational mode for cyclic BmImþ at

655 cm�1, 693 cm�1, 3109 cm�1 and 3159 cm�1 [33], CeH

bendingmode for cyclic BmImþ at 752 cm�1 [33], in-plane CeH

bending mode of imidazolium ring at 890 cm�1 [34], in-plane

CeNeC bending mode at 951 cm�1 [35], out-of-plane CeH

wagging mode in alkyl chain at 1019 cm�1 [34], CH3eN

stretching mode at 1168 cm�1 [36], CH2 symmetric bending

mode at 1335 cm�1 [33], CH3 asymmetric stretching mode at

1378 cm�1 [33], CH3 asymmetric bending mode at 1428 cm�1

and 1456 cm�1 [36], C]N stretching mode at 1540 cm�1 [37]

and CeC and CeN bending modes of imidazolium ring at

1696 cm�1 [33]. Some characteristic peaks of CL 0 are dis-

appeared with doping of BmImCl. Examples of these charac-

teristic peaks are the combinations of CHOH bending mode,

CH3 in-plane deformation and CeH wagging mode at

Fig. 4 e ATReFTIR spectra of (a) CL 0, (b) BmImCl, (c) Cl 2, (d)

CL 5 and (e) CL 6.


1329 cm�1 cm�1, CeH deformationmode at 1414 cm�1 and C]

O stretching mode at 1701 cm�1.

Apart from that, the CeCl stretching mode of BmImCl at

791 cm�1 is disappeared. This entails the chloride dissociation

from BmIm cations. Therefore, the dissociated BmIm cations

could form the carbene compound after the deprotonation

process, as suggested in section 3.1. This idea is supported by

the absences of out-of-plane CeH bending mode of imidazo-

lium ring at 730 cm�1 [35], CeH bendingmode inmethyl group

at 1132 cm�1 [38], CeN stretchingmode of imidazolium ring at

1289 cm�1 [35] and ¼ CeH stretching mode at 3091 cm�1 [35].

The proton conduction in ammonium salt-doped polymer

electrolyte is based on the formation of CONH bond, as

explained in section 3.1. However, the proton is migrated in a

different way for polymer electrolytes containing BmImCl, as

suggested in section 3.1. The CONH bonding mode of CL 0 is

absent in all the ionic liquid-based polymer electrolytes and

further verifies the proposed mechanism.

The CeH bending mode of CL 0 is shifted to lower wave-

number from 662 cm�1 to 655 cm�1 due to the overlapping of

CeH vibration mode for cyclic BmImþ. The CeH vibration

mode for cyclic BmImþ at 698 cm�1 also exhibit downward

shift to 693 cm�1 by comparing Fig. 4(a) with Fig. 3(d). The

sharp peak at 844 cm�1 in Fig. 4(a) is designated as skeletal

CeH rockingmode of PVA and shifted to 846 cm�1 with doping

of ionic liquid. This peak not only exhibits the change in shift,

but it also undergoes the change in peak intensity. The in-

tensity of peak is gradually reduced from 11.28% to 8.66%, in

transmittance mode, as demonstrated in Fig. 5. This phe-

nomenon indicates the interaction between ionic liquid and

polymer matrix. Another apparent proof to show the inter-

action between CeH bonding in imidazolium ring and CL 0 is

also attained at the wavenumber of 890 cm�1. A sharp peak

with its intensity of 19.12% is observed in ionic liquid spectra;

however it has been turned into a weak peakwith its intensity

of 0.14% in transmittance mode. Fig. 4(b) depicts a sharp peak

at 900 cm�1 with a shoulder peak at 940 cm�1. Upon addition

of PVA and ammonium acetate, this band becomes two new

peaks with an extremely low intensity at 918 cm�1 and

951 cm�1. These twoweak peaks are assigned as OeH bending

mode and in-plane CeNeC bending mode at 918 cm�1 and

951 cm�1, respectively, reflecting the formation of NO bond as

aforementioned in section 3.1 [28]. The intensity of first peak

is also reduced from 3.35% to 1.06% in transmittance mode. A

sharp peak is observed at 1089 cm�1 in the wavenumber range

of 1100 cm�1e1000 cm�1 for CL 0 and assigned as CeO

stretching mode. But, it has been changed to a medium sharp

peak at 1092 cm�1 with a shoulder peak at 1021 cm�1 when

BmImCl is embedded into the polymer complex. This in-

dicates the overlapping of out-of-plane CeH wagging mode in

alkyl chain with polymer matrix and further deduces the

interaction between CeO bond from polymer matrix and CeH

functional group from ionic liquid. This result agreeswell with

our proposed mechanism in section 3.1 where the hydrogen

from ionic liquid will be abstracted to the adjacent electron-

deficient oxygen for the proton hopping mechanism. Even

though the CeO stretching mode still remains unchanged at

1236 cm�1, however, the intensity of the peak is relatively

reduced upon impregnation of ionic liquid. The peak reduc-

tion entails that the proton conduction could be taken place at




Table 1 e The assignments of the vibration modes of BmImCl, CL 0, CL 2, CL 5 and CL 6.

Descriptions of vibration modes Wavenumber (cm�1)

BmImCl CL 0 CL 2 CL 5 CL 6

CeH bending mode e 662 655 655 657

CeH vibration mode for cyclic BmImþ 662, 698,

3113, 3152

e 655, 698,

3103, 3159

655, 693,

3109, 3159

657, 703,

e, e

Outeofeplane CeH bending mode of

imidazolium ring

730 e e e e

CeH bending mode for cyclic BmImþ 746 e 753 752 751

CeCl stretching mode 791 e e e e

Skeletal CeH rocking mode e 844 846 846 848

Ineplane CeH bending mode of imidazolium

ring

900 e 885 890 e

OeH bending mode e 918 918 918 918

Ineplane CeNeC bending mode 940 e 951 951 e

Outeofeplane CeH wagging mode in alkyl

chain

1018 e 1019 1021 1019

CeO stretching mode e 1089 1092 1092 1089

CeH bending mode in methyl group 1132 e e e e

CeC and CeO stretching modes of doubly H

ebonded OH in crystalline region

e 1140 1143 1140 1142

CH3eN stretching mode 1163 e 1169 1168 e

CeO stretching mode e 1236 1236 1236 1236

CeN stretching mode of imidazolium ring 1289 e e e e

CHOH bending mode, CH3 ineplane

deformation and CeH wagging mode

e 1329 e e e

CH2 symmetric bending mode 1337 e 1335 1335 1332

CH3 asymmetric stretching mode 1388 e 1378 1378 1375

CeH deformation mode 1413 1414 1409 e 1410

CH3 asymmetric bending mode 1434 e e 1428 e

CH3 symmetric bending mode 1465 e 1453 1456 e

C¼C stretching mode 1517 e 1504 e 1504

C¼N stretching mode 1560 e e 1540 e

NeH bending mode e 1561 1567 1571 1559

CeH stretching mode e 1648 1648 1647 1652

eCONHe bonding mode e 1669 e e e

CeC and CeN bending mode of imidazolium

ring

1675 e 1696 1696 1699

C¼O stretching mode e 1701 e e e

CeH symmetric stretching mode in methyl

group of alkyl chain

2872, 2950,

2966

2850, 2906,

2937

2876, 2911,

2940

2870, 2939 2850, 2911,

2938

OeH stretching mode of OH group e 3259 3332 3347 3260

¼CeH stretching mode 3091 e e e e


the CeO coordination bond when the ionic liquid is com-

plexed with polymer backbone.

The sharp peak at 1163 cm�1 is designated as CH3eN

stretching mode, whereas the weak peak at 1388 cm�1 is

associated with CH3 asymmetric stretching mode. The first

peak exhibits upward shift to 1168 cm�1 with reduced peak

intensity of 31.11%, from 44.11% to 13% in transmittance

mode. On the contrary, the latter peak shifts to lower wave-

number to 1378 where its intensity is gradually reduced from

3.17% to 0.73% in transmittance mode. The peak at 1414 cm�1

is designated as CeH deformation mode. However, the in-

clusion of 50 wt% of ionic liquid induces to the formation

double peaks at 1456 cm�1 and 1428 cm�1, as shown in Fig. 6.

The change in shape is suggestive of the overlap of CeH

deformation mode at 1413 cm�1, CH3 asymmetric bending

mode at 1434 cm�1 and CH3 symmetric bending mode at

1465 cm�1. This can be proven using the deconvolution

method. The peaks are fitted and deconvoluted as illustrated


in Fig. 7. Three deconvoluted peaks are obtained in this

wavenumber range. This result is in good agreement with our

suggestion. Again, this signifies the interaction of hydrogen

from CeH bond with polymer membrane where the proton

conduction can take place at. A medium sharp peak at

1561 cm�1 corresponding to NeH bending mode is attained in

CL 0 spectra. Nevertheless, the peak has been developed into a

sharp peak at 1571 with a shoulder peak at 1540 cm�1. The

intensity of the sharp peak is increased from 6.56% to 10.01%

in transmittance mode. The overlapping of C]N stretching

mode at 1560 cm�1 andC]C stretchingmode from ionic liquid

at 1517 cm�1 contributes to this changes of peak intensity,

peak location and shape. This can be a sign of interaction

between ionic liquid and CL 0. The weak peak analogous to

CeH stretching mode at 1648 cm�1 is changed to a medium

sharp peak at 1647 cm�1. The intensity of this peak is

increased from 0.43% to 5.63% in transmittance mode as

observed in Fig. 4(a and d). This finding reveals the




Fig. 5 e The change in peak intensity of skeletal CeH

rocking mode of (a) CL 0 and (b) CL 5 in the wavenumber

region of 850 cmL1e800 cmL1.

Fig. 7 e The original and fitted curves with the

deconvoluted peaks in the wavenumber region of

1495 cmL1e1390 cmL1.


complexation between ionic liquid and macromolecules and

verifies the proton conduction.

Another proof to show the proton conduction is also

detected in the range of 3000 cm�1e2800 cm�1. Double peaks

with a shoulder peak are appeared as a broad bandwithin this

region. This band is designated as CeH symmetric stretching

mode in methyl group of alkyl chain. However, only a peak

with a shoulder peak is observed at their respective wave-

number of 2939 cm�1 and 2870 cm�1 upon inclusion of

BmImCl. Similar phenomenon is attained for the band within

the range of 3400 cm�1e3000 cm�1. CL 0 illustrates a broad

peak at 3259 which is related to OeH stretching vibration

mode of OH group in PVA. Two additional shoulder peaks are

scrutinized at 3109 cm�1 and 3159 cm�1 in which the OeH

-0.01

0

0.01

0.02

0.03

0.04

0.05

1380 1400 1420 1440 1460 1480 1500

Abs

orba

nce

Wavenumber (cm-1)

Original curve Fitted curve Deconvoluted peaks

1465 cm-1

1434 cm-1

1413 cm-1

Fig. 6 e The change in shape of vibration modes of (a) CL

0 and (b) CL 5 in the wavenumber region of

1500 cmL1e1400 cmL1.


stretching mode is shifted to 3347 cm�1 with doping of

BmImCl. These newly formed shoulder peaks are known as

CeH vibration mode for cyclic BmImþ of ionic liquid. There-

fore, these changes in shift and peak position can be indicative

of interaction between CeH vibrationmode of ionic liquid and

OeH stretching mode of PVA which aids in the proton

conduction.

Among all the samples, we found out that the highest ionic

conductivity is achieved by adding 50 wt% of BmImCl as

shown in Fig. 2. One of the attributors in enhancing the ionic

conductivity is correlated to lower crystalline region (or higher

amorphous degree) in the polymer electrolytes. The vital

feature can be proven in the ATR-FTIR spectra. CeC and CeO

stretchingmodes of doubly H-bonded OH in crystalline region

is located at 1140 cm�1 with its intensity of 5.57% in Fig. 4(a).

The intensity of this peak is gradually reduced to 3.97% cor-

responding to CL 2 at 1143 cm�1 and 1.61% at 1140 cm�1 for CL

5. Therefore, it can be concluded that CL 5 has the lowest

crystalline region compared to CL 0 and CL 2. It is in good

agreement with the suggestion that we proposed in section

3.1. The intensity of this characteristic peak of CL 6 is

increased to 6.42% in comparison to CL 0, CL 2 and CL 5, as

shown in Fig. 4(e). When the ionic liquid is added further, the

crystalline phase of CL 6 becomes higher due to the ion pairing

and ion aggregation as a result of excessive ion. The formation

of ion pairs and ion aggregates can be further proven by the

disappearance of some characteristic peaks in Fig. 4(e). These

peaks are: in-plane CeH bending mode of imidazolium ring at

900 cm�1, in-plane CeNeC bending mode at 940 cm�1, CH3eN

stretchingmode at 1163 cm�1, CH3 asymmetric bendingmode

at 1434 cm�1, CH3 symmetric bending mode at 1465 cm�1, C]

N stretching mode 1560 cm�1, and CeH vibration modes for

cyclic BmImþ at 3113 cm�1 and 3152 cm�1. The changes in

peak intensity, peak position and shape establish the

complexation between ionic liquid and polymer matrix.




Fig. 9 e The power density of the fabricated PEMFC

containing CL0, CL 2, CL 5 and CL 6 polymer electrolytes at

different current density with inset of figure.


3.4. Cell performances

In order to study the effect of ionic liquid onto the electro-

chemical performances of fuel cell, four fuel cells are assem-

bled by sandwiching CL0, CL 2, CL 5 or CL 6 between anode and

cathode and further analyzed. Electrolyzer is used to generate

hydrogen and oxygen gases. The hydrogen gas is connected to

anode, whereas the oxygen is channeled to cathode. Below are

the general reactions in both electrodes.

Anode : 2H2/4Hþ þ 4e�

Cathode : 4Hþ þO2/2H2O

Complete reaction : 2H2 þO2/2H2O

Hydrogen gas which acts as fuel will be split into positively

charged hydrogen (or known as proton) and negatively

charged electrons in the presence of catalyst layer at anode.

For next step, the polymer electrolyte membrane allows the

protons to travel from anode to cathode. In contrast, the

electrons are transported along the external circuit to cathode

and hence create electrical current. Eventually, the combina-

tions between proton and oxygen with the help of electrons

will formwater as by-product at cathode side. This by-product

will be flowed out from the system.

The open-circuit voltage (Voc) is determined when no cur-

rent flows into the fuel cell. This is known as the initial voltage

prior to the cell voltage measurement. Figs. 8 and 9 depict the

cell potential and power density of the electrochemical cells

with different current density at room temperature, respec-

tively. The Voc of CL 0, CL 2, CL 5 and CL 6 are 0.27 V, 0.37 V,

0.66 V and 0.56 V, respectively as illustrated in Fig. 8. As can

been seen in current density-potential curves, the cell po-

tential of ionic liquid-based polymer electrolytes are higher

than that of polymer electrolytes without inclusion of ionic

liquid. This is mainly attributed to the strong plasticizing ef-

fect of ionic liquid. The plasticizing effect of ionic liquid not

only helps in promoting the proton conduction by weakening

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 10 20 30 40 50 60 70

Cel

l pot

enti

al (

V)

Current density (mAcm-2)

CL 0 CL 2 CL 5 CL 6

Fig. 8 e The cell potential of the fabricated PEMFC

containing CL0, CL 2, CL 5 and CL 6 polymer electrolytes at

different current density.


the transient bonds, but it also produces sticky samples. The

sticky feature of the sample can improve the contact between

the cathode and anode. Therefore, the proton can be traveled

easily. All the curves manifest the same pattern with respect

to current density. The cell potential of fuel cell decreases

with increasing the current density. The result is same as the

findings that other researchers have reported previously

[3e6]. Ionic liquid-based polymer electrolytes possess better

electrochemical performance than CL 0 by showing higher

current density at the end of the curve. This occurrence in-

dicates that fuel cell comprising of ionic liquid-based polymer

electrolytes can withstand higher current before the complete

degradation process. CL 0 has been fully decomposed when

620 mA of current (or equivalent with current density of

38.75mA cm�2) is applied into the cell. Upon inclusion of 20wt

% of ionic liquid into the polymer electrolyte, themembrane is

degraded at the current of 660 mA (or corresponding to

41.25 mA cm�2). On the other hand, the fuel cell consisting CL

5 stops the function at the current 950 mA or current density

of 59.34 mA cm�2. However, the electrochemical properties of

fuel cell are decreased when BmImCl is further added into the

polymermatrix. Comparing CL 5 and CL 6, CL 6 is decomposed

at lower current that is 700 mA with current density of

43.75 mA cm�2.

Power density of fuel cell is also determined. The power

density of fuel cell increases with current density, up to a

maximum level, as depicted in Fig. 9. The inset of figure por-

trays the peak obtained for fuel cell based on CL 0, CL 2 and CL

5 in a smaller scale. Beyond the highest value, the power

density of fuel cell is found to be decreased gradually. This

decrement of power density is correlated to the degradation of

polymer electrolytes at higher current supply. The doping of

ionic liquid can enhance the electrochemical properties of fuel

cell as explained above. The same conclusion can also be

drawn using the power density result. Fuel cell based on CL 2





polymermembrane exemplifies higher power density than CL

0. The maximum power density of 0.73 mW cm�2 is observed

at 0.05 V with a peak current density of 15.63 mA cm�2. This

peak is shifted to higher current with a higher power density

with addition of 20 wt% of BmImCl. Fuel cell comprising of CL

2 displays a maximum power density of 1.13 mW cm�2 at a

maximum potential of 0.06 V with a peak current density of

18.13 mA cm�2. Among all the ionic liquid-based polymer

electrolytes, the fabricated fuel cell using CL 5 offers excellent

electrochemical characteristics, where the power density of

18mWcm�2 at a peak current density of 46.88mA cm�2 with a

maximum operational potential of 0.38 V is observed in Fig. 9.

The increase in power density of fuel cell as increases the

mass loading of ionic liquid into the polymer matrix is

strongly due to the higher ionic conductivity of ionic liquid-

based polymer electrolytes. Numerous protons with higher

mobility are allowed to be transported from anode to cathode

if the polymer electrolyte is conductive. Therefore, higher

conductive characteristic of polymer electrolyte promotes the

conversion of the fuel into electricity through a redox process

in a fuel cell system and thereby increases the power density

of the electrochemical cell. Nevertheless, the further inclusion

of ionic liquid alters the electrochemical properties of fuel cell.

Fuel cell based on CL 6 exhibits the maximum power density

of 2.96mWcm�2 at amaximumpotential of 0.25 Vwith a peak

current density of 11.88 mA cm�2 is observed in Fig. 9. Upon

addition of 60 wt% of BmImCl not only decreases the power

density of cell, but it also reduces the peak current density, as

depicted in the inset of Fig. 9. In other words, excessive ionic

liquid concentration in the polymer electrolytes could pro-

duce the ion aggregation which inhibits the conducting routes

within the polymer chains. As a result, this proton con-

ducting-polymer electrolyte possesses lower ionic conduc-

tivity and hence leads to lower electrochemical performance

of fuel cell.

Composite polymer electrolytes-based direct methanol

fuel cells (DMFCs) were also prepared by Yang group. The

maximumpower density of 6.77mWcm�2 was reported in the

literature. Another type of DMAFC using crosslinked PVA/SSA

was also prepared by Yang. This DMAFC generates the highest

power density of 4.13 mW cm�2 at 60 �C and 1 atm [39,40]. The

power density of PEMFC obtained in this present work is

higher than these two literature and the literature that we

mentioned in the introduction. So, this implies that the pre-

pared ionic liquid-based proton conducting polymer electro-

lytes exhibit excellent electrochemical performance in fuel

cell. It can be concluded that CL 5 is the promising candidate

as polymer electrolyte in a fuel cell as it can achieves the

highest cell power density of 18 mW cm�2 with improved

electrochemical properties.

4. Conclusion

Ionic liquid-based proton conducting polymer electrolytes

were prepared by solution casting method. The ionic con-

ductivity of these polymer electrolytes is greatly increased

with doping of ionic liquid due to the strong plasticizing effect.

The ionic conductivity also increased with mass fraction of

ionic liquid, up to a maximum level, which corresponds to


inclusion of 50 wt% of BmImCl. Upon addition of 50 wt% of

BmImCl, the highest ionic conductivity of (5.74 � 0.01)

mS cm�1 is achieved at ambient temperature. This most

conductive polymer membrane exhibited better thermal

properties than other polymer electrolytes as it is stable up to

250 �C. The temperature of first thermal degradation stage is

shifted to higher temperature with doping of salt and ionic

liquid due to the complexation. ATR-FTIR showed the in-

teractions between PVA, ammonium acetate and BmImCl and

further proved the complexation within the polymer electro-

lytes. Fuel cells were eventually fabricated using the electro-

lyzer and fuel cell membrane kit. Ionic liquid also helped in

enhancing the electrochemical properties of fuel cells. The

open-circuit voltage and power density of fuel cell have been

greatly improved with doping of ionic liquid. The maximum

power density of 18 mW cm�2 was achieved in the fuel cell

containing the most conducting polymer electrolyte at room

temperature, along with operational current of 750 mA.

Acknowledgment

This work was supported by the High Impact Research Grant

(J-21002-73851) from University of Malaya and eScienceFund

(16e02e03e6031) from Ministry of Science, Technology and

Innovation (MOSTI), Malaysia. Gratitude also goes to “Perun-

tukan Penyelidikan Pascasiswazah (PPP), Universiti Malaysia”.

One of the authors, ChiameWen Liew gratefully acknowl-

edges the “Skim Bright Sparks Universiti Malaya” (SBSUM) for

scholarship award.

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