A novel approach on ionic liquid-based PVA

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A novel approach on ionic liquid-based poly(vinyl
alcohol) proton conductive polymer electrolytes for
fuel 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
article info
abstract
Article history:
The preparation of proton conducting-polymer electrolytes based on poly(vinyl alcohol)
Received 8 December 2012
(PVA)/ammonium acetate (CH3COONH4)/1-butyl-3-methylimidazolium chloride (BmImCl)
Received in revised form
was done by solution casting technique. The ionic conductivity increased with ionic liquid
4 July 2013
mass loadings. The highest ionic conductivity of (5.74 0.01) mS cm1 was achieved upon
Accepted 23 July 2013
addition of 50 wt% of BmImCl. The thermal characteristic of proton conducting-polymer
Available online 17 August 2013
electrolytes is enhanced with doping of ionic liquid by showing higher initial decomposition temperature. The most conducting polymer electrolyte is stable up to 250 C. Attenuated total reflectance-Fourier Transform Infrared (ATR-FTIR) confirmed the complexation
Keywords:
PVA
Ionic liquid
Proton conductivity
between PVA, CH3COONH4 and BmImCl. Polymer electrolyte membrane fuel cell (PEMFC)
was fabricated. This electrochemical cell achieved the maximum power density of
18 mW cm2 at room temperature.
Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
Fuel cell
reserved.
Power density
1.
Introduction
Depletion of non-renewable fossil fuel catalyzes the development of environmentally friendly alternative energy sources. 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 energy 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
component in PEMFCs is the proton exchange membrane
(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 monoxide (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
* Corresponding author. Tel.: þ60 3 7967 4391; fax: þ60 3 7967 4146.
E-mail addresses: [email protected] (C.-W. Liew), [email protected] (S. Ramesh), [email protected] (A.K. Arof).
0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.ijhydene.2013.07.092
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[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 Nafion membrane 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]. Furthermore, 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 storage which is an important feature for a
supercapacitor, as reported by Hirankumar et al. [15]. In order
to prepare proton-conducting polymer electrolytes, ammonium 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, impregnation 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 substances 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-1propanesulfonic acid (AMPS). The maximum power density of
2.4 mW cm2 was delivered by this DMFC [7]. In addition, PVAbased 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 cm2 was achieved
using PVA/potassium hydroxide (KOH) with doping of 20% of
fumed silica [6]. However, lower maximum power density of
7.54 mW cm2 was obtained in PVA/titania (TiO2)-based DMFC
[4]. Sekhon et al. synthesized polymer electrolyte containing
poly(vinylidene fluoride-co-hexafluoropropylene (PVdF-coHFP), triflic acid (HCF3SO3) and 2,3-dimethyl-1-octylimidazolium triflate (DmOImTf) for a PEMFC. Power density of
0.5 mW cm2 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 performances of PEMFCs such as power density are eventually
studied in the present work. Another aim of this current
project is to explore the effect of ionic liquid onto the electrochemical 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 mol1), 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 technique. PVA was initially dissolved in distilled water. Appropriate amount of CH3COONH4 was subsequently mixed in PVA
solution. The weight ratio of PVA:CH3COONH4 was kept at
70:30. Different mass fraction of BmImCl was thus embedded
into the PVA-CH3COONH4 mixture to prepare ionic liquidbased 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 incorporation of BmImCl. The resulting solution was stirred thoroughly 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 proton
conducting polymer electrolytes
2.3.1.
Ambient temperature-ionic conductivity studies
Freshly 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 electrolytes was measured by HIOKI 3532-50 LCR HiTESTER impedance analyzer over a frequency regime between 50 Hz and
5 MHz at ambient temperature. The data acquisition was
recorded at a signal level of 10 mV. 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 samples were then heated from 25 C to 600 C at
a heating rate of 50 C min1 in a nitrogen atmosphere with a
flow rate of 60 ml min1.
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2.3.3. Attenuated total reflectance-Fourier Transform
Infrared (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 cm1 in transmittance mode over the wavenumber range from 4000 to
650 cm1. The FTIR spectra and peak deconvolution were
scrutinized by OMNIC 8 software which 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 implemented. 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 Electrolyzer 10 (from H-tec, Germany) for electrochemical analysis
with hydrogen flow rate of 10 cm3 min1 and oxygen flow rate
of 5 cm3 min1.
2.4.1.
Cell performances
The performance of PEMFC was done by fuel cell measurement 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 cm2). On the other hand, the power
density was calculated using the fundamental equation below:
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.
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 extrapolation 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 rapidly with doping
of 20 wt% of BmImCl. The highest ionic conductivity of
(5.74 0.01) mS cm1 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 electrolyte without impregnation of ionic liquid. The abrupt increase 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
400
350
300
250
200
150
100
Rs
50
Rs+Rb
0
0
3.
Results and discussion
3.1.
Ambient temperature-ionic conductivity studies
50
100
150
200
250
300
350
The ionic conductivity of polymer electrolytes is determined
using the following equation:
s¼
l
Rb A
(2)
Fig. 1 e ColeeCole impedance plot of CL 0 at room
temperature.
400
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-2.00
H
-2.50
..
Deprotonation
+
N
N
N
+
+H
N
-3.00
-3.50
-4.00
This carbene could then interact with the hydrogen in
hydroxyl group of PVA and result carbocation in the imidazolium ring.
-4.50
-5.00
-5.50
0
10
20
30
40
50
60
70
Weight percent of BmImCl (wt %)
80
90
CH
CH2
Fig. 2 e The ionic conductivity of proton conducting
polymer electrolytes at different mass ratio of BmImCl.
N
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 electrolyte 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-conducting 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: imidazolium 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].
CH
H
C
CH
H
CH2
N
N
N
CH
O
OH
OH
n
C
N
This interaction leads to the partial hydrogen bonding
between oxygen and hydrogen. In addition, the de-protonated 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 oxygen 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 imidazolium cations. Two possible side products could be formed in
this segment, viz. 3-butyl-2-chloro-1-methyl-4,5-dihydro-1Himidazole and 3-butyl-1-methyl-4,5-dihydro-1H-imidazole-2carboxylic acid methyl ester, as shown in the mechanism
below.
3-butyl-2-chloro-1-methyl-4,5-dihydro-1H-imidazole
CH2
CH
+
Cl
Cl or / and CH3COO
+
CH
H
n
OH
O
N
CH2
n
OH
O
..
CH2
CH
CH2
CH
:O
..:
CH2
CH
n
+
N
OH
N
or / and
OOCCH3
N
N
3-butyl-1-methyl-4,5-dihydro-1H-imidazole-2-carboxylic acid methyl ester
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cations into immobile state. As a consequence, the conducting
sites could be lesser and ultimately reduce the ionic conductivity. 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 determining 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 transportation. Therefore, we can conclude that the lower ionic
conductivity of the polymer electrolytes containing high mass
fraction of BmImCl is attributed to the formation of ion pairs
Beyond this step, there are two possibilities of proton
conduction process. The proton transportation can be generated 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 coordination 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.
CH
CH2
H
CH
CH2
:..
O:
+
O
CH
CH2
n
O
H
CH2
CH
n
:O
..:
H
Cl or / and CH3COO
and ion aggregates as the ion pairs can block the transport
pathway within the polymer chains.
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 electrondeficient 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.
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..
CH2
CH
:O
..:
N
+
CH2
CH
O
n
H
N
CH2
CH
CH2
O H
N
CH
n
+
N
N
:O
..:
N
Cl or / and CH3COO
Beyond adulteration of 50 wt% of BmImCl, the ionic conductivity is found to be lower in value. This is ascribed to the
agglomeration of ions. Excessive ions in the polymer electrolyte 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, NHþ
4 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 aforementioned above. However, at high mass loadings, the cations
could form ion neutral pairs with anions and thus turn the
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 acetamide, CH3C(O)NH2.
On the contrary, two PVA polymer chains might be crosslinked 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
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backbone of PVA could start a rapid chain stripping on the
polymer backbone upon heating and hence turn into conjugated double bonds, giving rise to formation of polyene, as
illustrated as below [11]:
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 agreement with 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 temperature starts from 305 C to 355 C. The thermal degradation 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
Upon addition of salt and ionic liquid, the mass loss in this
stage is significantly decreased. Besides, the starting decomposition temperature in this stage is improved. These observations deduct the contribution from the complexation
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].
between salt and/or ionic liquid and PVA. This is because
higher energy would be required to break the transient coordination 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
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. Ionic
liquid-based polymer electrolytes display lower mass loss in
comparison to CL 0. CL 2 exhibits 21% of weight loss from
365 C to 570 C with residual mass 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, the weight
120
100
Weight (%)
80
60
40
20
0
0
100
200
Pure PVA
300
CL 0
400
CL 2
CL 5
500
600
CL 6
Fig. 3 e Thermogravimetric analysis of pure PVA, CL 0, CL
2, CL 5 and CL 6.
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 e n e r g y 3 9 ( 2 0 1 4 ) 2 9 1 7 e2 9 2 8
of samples remains unchanged. This observation infers the
fully decomposition of the polymer matrix. Although the residual 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 Transform
Infrared (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 cm1, 693 cm1, 3109 cm1 and 3159 cm1 [33], CeH
bending mode for cyclic BmImþ at 752 cm1 [33], in-plane CeH
bending mode of imidazolium ring at 890 cm1 [34], in-plane
CeNeC bending mode at 951 cm1 [35], out-of-plane CeH
wagging mode in alkyl chain at 1019 cm1 [34], CH3eN
stretching mode at 1168 cm1 [36], CH2 symmetric bending
mode at 1335 cm1 [33], CH3 asymmetric stretching mode at
1378 cm1 [33], CH3 asymmetric bending mode at 1428 cm1
and 1456 cm1 [36], C]N stretching mode at 1540 cm1 [37]
and CeC and CeN bending modes of imidazolium ring at
1696 cm1 [33]. Some characteristic peaks of CL 0 are disappeared with doping of BmImCl. Examples of these characteristic 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.
2923
1329 cm1 cm1, CeH deformation mode at 1414 cm1 and C]
O stretching mode at 1701 cm1.
Apart from that, the CeCl stretching mode of BmImCl at
791 cm1 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 imidazolium ring at 730 cm1 [35], CeH bending mode in methyl group
at 1132 cm1 [38], CeN stretching mode of imidazolium ring at
1289 cm1 [35] and ¼ CeH stretching mode at 3091 cm1 [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 wavenumber from 662 cm1 to 655 cm1 due to the overlapping of
CeH vibration mode for cyclic BmImþ. The CeH vibration
mode for cyclic BmImþ at 698 cm1 also exhibit downward
shift to 693 cm1 by comparing Fig. 4(a) with Fig. 3(d). The
sharp peak at 844 cm1 in Fig. 4(a) is designated as skeletal
CeH rocking mode of PVA and shifted to 846 cm1 with doping
of ionic liquid. This peak not only exhibits the change in shift,
but it also undergoes the change in peak intensity. The intensity of peak is gradually reduced from 11.28% to 8.66%, in
transmittance mode, as demonstrated in Fig. 5. This phenomenon indicates the interaction between ionic liquid and
polymer matrix. Another apparent proof to show the interaction between CeH bonding in imidazolium ring and CL 0 is
also attained at the wavenumber of 890 cm1. A sharp peak
with its intensity of 19.12% is observed in ionic liquid spectra;
however it has been turned into a weak peak with its intensity
of 0.14% in transmittance mode. Fig. 4(b) depicts a sharp peak
at 900 cm1 with a shoulder peak at 940 cm1. Upon addition
of PVA and ammonium acetate, this band becomes two new
peaks with an extremely low intensity at 918 cm1 and
951 cm1. These two weak peaks are assigned as OeH bending
mode and in-plane CeNeC bending mode at 918 cm1 and
951 cm1, 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 cm1 in the wavenumber range
of 1100 cm1e1000 cm1 for CL 0 and assigned as CeO
stretching mode. But, it has been changed to a medium sharp
peak at 1092 cm1 with a shoulder peak at 1021 cm1 when
BmImCl is embedded into the polymer complex. This indicates 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 agrees well with
our proposed mechanism in section 3.1 where the hydrogen
from ionic liquid will be abstracted to the adjacent electrondeficient oxygen for the proton hopping mechanism. Even
though the CeO stretching mode still remains unchanged at
1236 cm1, however, the intensity of the peak is relatively
reduced upon impregnation of ionic liquid. The peak reduction entails that the proton conduction could be taken place at
2924
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 e n e r g y 3 9 ( 2 0 1 4 ) 2 9 1 7 e2 9 2 8
Table 1 e The assignments of the vibration modes of BmImCl, CL 0, CL 2, CL 5 and CL 6.
Wavenumber (cm1)
Descriptions of vibration modes
BmImCl
CeH bending mode
CeH vibration mode for cyclic BmImþ
Outeofeplane CeH bending mode of
imidazolium ring
CeH bending mode for cyclic BmImþ
CeCl stretching mode
Skeletal CeH rocking mode
Ineplane CeH bending mode of imidazolium
ring
OeH bending mode
Ineplane CeNeC bending mode
Outeofeplane CeH wagging mode in alkyl
chain
CeO stretching mode
CeH bending mode in methyl group
CeC and CeO stretching modes of doubly H
ebonded OH in crystalline region
CH3eN stretching mode
CeO stretching mode
CeN stretching mode of imidazolium ring
CHOH bending mode, CH3 ineplane
deformation and CeH wagging mode
CH2 symmetric bending mode
CH3 asymmetric stretching mode
CeH deformation mode
CH3 asymmetric bending mode
CH3 symmetric bending mode
C¼C stretching mode
C¼N stretching mode
NeH bending mode
CeH stretching mode
eCONHe bonding mode
CeC and CeN bending mode of imidazolium
ring
C¼O stretching mode
CeH symmetric stretching mode in methyl
group of alkyl chain
OeH stretching mode of OH group
¼CeH stretching mode
CL 0
CL 2
CL 5
CL 6
655
655, 693,
3109, 3159
e
657
657, 703,
e, e
e
e
662, 698,
3113, 3152
730
e
655
655, 698,
3103, 3159
e
746
791
e
900
e
e
844
e
753
e
846
885
752
e
846
890
751
e
848
e
e
940
1018
918
e
e
918
951
1019
918
951
1021
918
e
1019
e
1132
e
1089
e
1140
1092
e
1143
1092
e
1140
1089
e
1142
1163
e
1289
e
e
1236
e
1329
1169
1236
e
e
1168
1236
e
e
e
1236
e
e
1337
1388
1413
1434
1465
1517
1560
e
e
e
1675
e
e
1414
e
e
e
e
1561
1648
1669
e
1335
1378
1409
e
1453
1504
e
1567
1648
e
1696
1335
1378
e
1428
1456
e
1540
1571
1647
e
1696
1332
1375
1410
e
e
1504
e
1559
1652
e
1699
e
2872, 2950,
2966
e
3091
1701
2850, 2906,
2937
3259
e
e
2876, 2911,
2940
3332
e
e
2870, 2939
e
2850, 2911,
2938
3260
e
the CeO coordination bond when the ionic liquid is complexed with polymer backbone.
The sharp peak at 1163 cm1 is designated as CH3eN
stretching mode, whereas the weak peak at 1388 cm1 is
associated with CH3 asymmetric stretching mode. The first
peak exhibits upward shift to 1168 cm1 with reduced peak
intensity of 31.11%, from 44.11% to 13% in transmittance
mode. On the contrary, the latter peak shifts to lower wavenumber to 1378 where its intensity is gradually reduced from
3.17% to 0.73% in transmittance mode. The peak at 1414 cm1
is designated as CeH deformation mode. However, the inclusion of 50 wt% of ionic liquid induces to the formation
double peaks at 1456 cm1 and 1428 cm1, as shown in Fig. 6.
The change in shape is suggestive of the overlap of CeH
deformation mode at 1413 cm1, CH3 asymmetric bending
mode at 1434 cm1 and CH3 symmetric bending mode at
1465 cm1. This can be proven using the deconvolution
method. The peaks are fitted and deconvoluted as illustrated
662
e
3347
e
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 cm1 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 cm1. 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 cm1 and C]C stretching mode from ionic liquid
at 1517 cm1 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 cm1 is changed to a medium
sharp peak at 1647 cm1. 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
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 e n e r g y 3 9 ( 2 0 1 4 ) 2 9 1 7 e2 9 2 8
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.
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 cm1e2800 cm1. Double peaks
with a shoulder peak are appeared as a broad band within 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 wavenumber of 2939 cm1 and 2870 cm1 upon inclusion of
BmImCl. Similar phenomenon is attained for the band within
the range of 3400 cm1e3000 cm1. 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 cm1 and 3159 cm1 in which the OeH
0.05
0.04
0.03
1465 cm-1
Absorbance
1434 cm-1
0.02
0.01
1413 cm-1
0
-0.01
1380
1400
1420
Original curve
1460
1440
Wavenumber (cm-1)
Fitted curve
1480
1500
Deconvoluted peaks
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.
2925
Fig. 7 e The original and fitted curves with the
deconvoluted peaks in the wavenumber region of
1495 cmL1e1390 cmL1.
stretching mode is shifted to 3347 cm1 with doping of
BmImCl. These newly formed shoulder peaks are known as
CeH vibration mode for cyclic BmImþ of ionic liquid. Therefore, these changes in shift and peak position can be indicative
of interaction between CeH vibration mode 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
stretching modes of doubly H-bonded OH in crystalline region
is located at 1140 cm1 with its intensity of 5.57% in Fig. 4(a).
The intensity of this peak is gradually reduced to 3.97% corresponding to CL 2 at 1143 cm1 and 1.61% at 1140 cm1 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 cm1, in-plane CeNeC bending mode at 940 cm1, CH3eN
stretching mode at 1163 cm1, CH3 asymmetric bending mode
at 1434 cm1, CH3 symmetric bending mode at 1465 cm1, C]
N stretching mode 1560 cm1, and CeH vibration modes for
cyclic BmImþ at 3113 cm1 and 3152 cm1. The changes in
peak intensity, peak position and shape establish the
complexation between ionic liquid and polymer matrix.
2926
3.4.
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 e n e r g y 3 9 ( 2 0 1 4 ) 2 9 1 7 e2 9 2 8
Cell performances
In order to study the effect of ionic liquid onto the electrochemical performances of fuel cell, four fuel cells are assembled 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 þ O2 /2H2 O
Complete reaction : 2H2 þ O2 /2H2 O
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 combinations between proton and oxygen with the help of electrons
will form water 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 current 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, respectively. 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 potential 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 effect of ionic liquid. The plasticizing effect of ionic liquid not
only helps in promoting the proton conduction by weakening
0.7
0.6
Cell potential (V)
0.5
0.4
0.3
0.2
0.1
0
0
10
20
30
40
50
Current density (mAcm-2)
CL 0
CL 2
CL 5
60
70
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.
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.
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 indicates 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
38.75 mA cm2) is applied into the cell. Upon inclusion of 20 wt
% of ionic liquid into the polymer electrolyte, the membrane is
degraded at the current of 660 mA (or corresponding to
41.25 mA cm2). 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 cm2. However, the electrochemical properties of
fuel cell are decreased when BmImCl is further added into the
polymer matrix. Comparing CL 5 and CL 6, CL 6 is decomposed
at lower current that is 700 mA with current density of
43.75 mA cm2.
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 portrays 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
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 e n e r g y 3 9 ( 2 0 1 4 ) 2 9 1 7 e2 9 2 8
polymer membrane exemplifies higher power density than CL
0. The maximum power density of 0.73 mW cm2 is observed
at 0.05 V with a peak current density of 15.63 mA cm2. 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 cm2 at a
maximum potential of 0.06 V with a peak current density of
18.13 mA cm2. Among all the ionic liquid-based polymer
electrolytes, the fabricated fuel cell using CL 5 offers excellent
electrochemical characteristics, where the power density of
18 mW cm2 at a peak current density of 46.88 mA cm2 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 liquidbased 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 2.96 mW cm2 at a maximum potential of 0.25 V with a peak
current density of 11.88 mA cm2 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 produce the ion aggregation which inhibits the conducting routes
within the polymer chains. As a result, this proton conducting-polymer electrolyte possesses lower ionic conductivity 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
maximum power density of 6.77 mW cm2 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 cm2 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 prepared ionic liquid-based proton conducting polymer electrolytes 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 cm2 with improved
electrochemical properties.
4.
Conclusion
Ionic liquid-based proton conducting polymer electrolytes
were prepared by solution casting method. The ionic conductivity 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
2927
inclusion of 50 wt% of BmImCl. Upon addition of 50 wt% of
BmImCl, the highest ionic conductivity of (5.74 0.01)
mS cm1 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 interactions between PVA, ammonium acetate and BmImCl and
further proved the complexation within the polymer electrolytes. Fuel cells were eventually fabricated using the electrolyzer 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 cm2 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 “Peruntukan Penyelidikan Pascasiswazah (PPP), Universiti Malaysia”.
One of the authors, ChiameWen Liew gratefully acknowledges the “Skim Bright Sparks Universiti Malaya” (SBSUM) for
scholarship award.
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