Download Incorporation of NH4Br in PVA-chitosan blend-based

Ionics (2014) 20:1235–1245
DOI 10.1007/s11581-014-1096-1
ORIGINAL PAPER
Incorporation of NH4Br in PVA-chitosan blend-based polymer
electrolyte and its effect on the conductivity and other electrical
properties
Y. M. Yusof & H. A. Illias & M. F. Z. Kadir
Received: 13 June 2013 / Revised: 24 January 2014 / Accepted: 20 February 2014 / Published online: 11 March 2014
# Springer-Verlag Berlin Heidelberg 2014
Abstract Polymer electrolyte system based on poly(vinyl
alcohol) (PVA)-chitosan blend doped with ammonium bromide (NH4Br) has been prepared by solution cast method.
Fourier transform infrared (FTIR) spectroscopy analysis confirms the complexation between salt and polymer host. The
highest ionic conductivity obtained at room temperature is
(7.68±1.24)×10−4 S cm−1 for the sample comprising of
30 wt% NH4Br. X-ray diffraction (XRD) patterns reveal that
PVA-chitosan with 30 wt% NH4Br exhibits the most amorphous structure. Thermogravimetric analysis (TGA) reveals
that the electrolytes are stable until ∼260 °C. The conductivity
variation can also be explained by field emission scanning
electron microscopy (FESEM) study. Dielectric properties of
the electrolytes follow non-Debye behavior. The conduction
mechanism of the highest conducting electrolyte can be represented by the correlated barrier hopping (CBH) model.
From linear sweep voltammetry (LSV) result, the highest
conducting electrolyte is electrochemically stable at 1.57 V.
Keywords Polymer electrolyte . Conductivity . PVA .
Chitosan . Ammonium bromide
This paper was presented at ICFMD 2013 in Penang, Malaysia.
Y. M. Yusof
Institute of Graduate Studies, University of Malaya, 50603 Kuala
Lumpur, Malaysia
H. A. Illias
Department of Electrical Engineering, Faculty of Engineering,
University of Malaya, 50603 Kuala Lumpur, Malaysia
M. F. Z. Kadir (*)
Centre for Foundation Studies in Science, University of Malaya,
50603 Kuala Lumpur, Malaysia
e-mail: [email protected]
Introduction
Polymer electrolytes have potential applications in electrochemical devices such as proton batteries and electrochemical
double layer capacitors (EDLCs) [1–4]. Solid polymer electrolytes (SPEs) are important materials for these devices that
require high performance with thin and flexible form factors
where the properties of the film are thermally and electrochemically stable [5]. Since conductivity is a crucial property
for polymer electrolyte, researchers have tried various
methods to enhance the conductivity. Blending two or more
polymers is one of the techniques to improve the conductivity
of polymer electrolytes as the host material for ionic conduction [6]. Polymer blend materials have attracted the attention
of many researchers because of its better mechanical properties and ease of fabrication [7].
PVA is a synthetic polymer with carbon chain backbone
attached with hydroxyl groups [8]. PVA is non-toxic, water
soluble, biocompatible and biodegradable synthetic polymer,
which is widely used in the biomedical field [9, 10]. PVA is also
used as host in polymer electrolyte systems [4, 8, 11]. Chitosan
is a copolymer of N-acetyl-D-glucosamine and D-glucosamine
(Fig. 1) [12]. In contrast with PVA, chitosan is a natural polymer
and its backbone consists of β-1,4-linked D-glucosamine with a
high degree of N-acetylation. Thus, chitosan is poly(N-acetyl-2amino-2-deoxy-D-glucopyranose), where the N-acetyl-2-amino-2-deoxy-D-glucopyranose or (Glu-NH2) units are linked
by (1→4)-β-glycosidic bonds [12]. Chitosan which is degraded by enzymatic hydrolysis is a semi-crystalline polymer and
the degree of crystallinity is a function of the degree of
deacetylation [13]. Buraidah et al. [14] reported that a polymer
electrolyte system prepared by doping chitosan with 45 wt%
NH4I produced an ionic conductivity of 3.73×10−7 S cm−1 at
room temperature. Chitosan containing 50 wt% ammonium
triflate (NH4CF3SO3) was reported to obtain a conductivity
value of 8.91×10−7 S cm−1 at room temperature [15].
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Ionics (2014) 20:1235–1245
a
homogenous solution was obtained. Different amounts of
NH4Br (Bendosen); 10–60 wt%; were added into the starchchitosan solutions and stirred until complete dissolution. All
solutions were cast into different plastic Petri dishes, left to dry
at room temperature (25 °C) and kept in a desiccator filled
with silica gel desiccants for further drying process.
H3C
C
O
H
H
H
CH2OH
H
NH
O
H
HO
O
HO
H
NH
H
H
C
H
O
O
CH2OH
H
Electrolytes characterization
O
H3C
b
H
H
CH2OH
O
H H
HO
O
HO
NH2
H
H
NH2
H
H
H
O
CH2OH
H
O
Fig. 1 Structure of a chitin and b chitosan
Blend systems using PVA hydrogel have been explored for
medical and pharmaceutical application due to the advantages
of non-toxic, non-carcinogenic and bioadhesive properties
[16]. Polymer blend complexes based on PVA and chitosan
are easy to prepare due to their highly controllable chemical
and physical properties, including toughness, miscibility
(homogeneity), and thermal stability [10, 17]. Chitosan and
PVA are compatible for blending since they are miscible into
each other [8]. This is caused by the formation of ionic
complexes between PVA and chitosan due to hydroxyl groups
in PVA and amine groups in chitosan. The strong interaction
will form a compatible one-phase blend of PVA and chitosan
[18]. From XRD diffractogram reported by Kadir et al. [4], the
blend of 60 wt% PVA and 40 wt% chitosan was found as the
most amorphous blend host. The present work aims at developing a new type of polymer electrolyte system based on the
blend of 60 wt% PVA and 40 wt% chitosan (3:2) where the
blend is expected to serve as a good polymer host. NH4Br was
chosen as the dopant since ammonium salts are considered as
good proton donor [19].
Experimental methods
Electrolytes preparation
The polymer electrolytes were prepared by solution cast method. A total of 0.6 g of PVA (degree of hydrolysis, 80 %;
molecular weight, 9,000–10,000; Sigma-Aldrich)] were dissolved in 100 mL of 1 % acetic acid solution (SYSTERM),
followed by the addition of 0.4 g of chitosan [viscosity, 800–
2000 cP; Sigma-Aldrich]. The mixtures were stirred until
The impedance measurements were conducted using HIOKI
3532-50 LCR HiTESTER in the frequency range between
50 Hz and 5 MHz from room temperature to 373 K. The
electrolyte films were sandwiched between two stainless steel
electrodes of a conductivity holder. The value of bulk resistance (Rb) obtained from the measurement was used to calculate the conductivity (σ) using the following equation
σ¼
t
ð1Þ
Rb A
where t is the thickness of the electrolytes and A is the
electrode/electrolyte contact area. The thicknesses of the electrolytes are listed in Table 1.
The FTIR studies were performed using Spotlight 400
Perkin-Elmer spectrometer in the wavenumber range of
400–4,000 cm−1 at a resolution of 1 cm−1. The objective of
performing FTIR studies was to conform the complexation
between polymer blend and salt. A Zeiss Auriga field emission scanning electron microscope at 1K× magnification was
used to study the morphology of the polymer electrolyte. This
study will give an insight of the surface morphology of the
semi-crystalline polymer electrolyte. XRD measurements of
the polymer blend electrolytes were carried out using Siemens
D5000 X-ray diffractometer where X-rays of 1.5406 Å wavelengths were generated by a Cu Kα source. The 2θ angle was
varied from 5° to 80°. In order to determine the thermal
stability, solid polymer electrolyte samples were subjected to
thermogravimetric (TGA) analysis. From the TGA thermograms, weight loss and phase transitions can also be studied.
TGA experiment has been carried out using Perkin-Elmer
Pyris 1 TGA equipment. The samples were heated from room
Table 1 Thickness of
PVA-chitosan-NH4Br
films at room
temperature
wt% NH4Br
Thickness (mm)
0
10
20
30
40
50
60
0.035±0.002
0.115±0.039
0.114±0.017
0.084±0.014
0.037±0.003
0.028±0.002
0.229±0.051
Ionics (2014) 20:1235–1245
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a
b
(v)
(viii)
1626
3083
2974
temperature to 800 °C at a heating rate of 10 °C min−1 under
helium atmosphere. The linear sweep voltammetry (LSV)
measurement was conducted using a three-electrode configuration where stainless steel electrodes were used as working,
counter, and reference electrodes as can be seen in Fig. 2. This
electrochemical stability window was measured using DigiIvy DY2300 potentiostat at a scan rate of 1 mV s−1 in a
potential range of 0 to 2.5 V. The LSV measurement was
conducted at room temperature.
(iv)
1628
The interactions of PVA-chitosan and PVA-chitosan-NH4Br
can be studied from FTIR spectroscopy. Blending two polymers can provide more sites for ion hopping and exchange
which lead to the increase in conductivity [8]. As reported by
Costa-Junior et al. [20], the bands observed within 2,800–
3,600 cm−1 region were associated with O–H and C–H
groups. The C–H band has shifted in the spectrum of PVAchitosan blend, refers to the stretching C–H from alkyl groups
[19]. Kadir et al. [21] reported that the hydroxyl band has
shifted from 3,343 cm −1 in PVA film spectrum and
3,354 cm−1 in chitosan film spectrum to 3,337 cm−1 in the
spectrum of pure PVA-chitosan film, which inferred that the
hydrogen bonds have been formed between hydroxyl groups
in PVA and amino or hydroxyl groups in chitosan [22]. The
region of the carboxamide and amine bands is observed within
1,490–1,680 cm−1, Fig. 3b(i), which are almost the same as
reported by Kadir et al. [21] and Buraidah and Arof [8]. This
work is also comparable to the work reported by Majid and
Arof [23] where the amine band of pure chitosan acetate film
appeared at 1,553 cm−1. Kadir et al. [21] reported that
carboxamide and amine bands of pure PVA-chitosan film
are situated at 1,647 and 1,558 cm−1, respectively.
Working
electrode
Reference
electrode
Counter
electrode
Fig 2 Linear sweep voltammeter (LSV) with three-electrode
configuration
3079
2934
(vi) 3293
(v) 3301
(iv) 3317
2881
2885
Transmittance (a.u.)
FTIR analysis
Transmittance (a.u.)
(vii)
Results and discussion
1508
1511
(iii)
1631
1513
(ii)
2893
1633
1517
(iii) 3324 2897
(i)
(ii) 3331
2901
(i) 3335
2903
1645
3600
3200
2800
-1
Wavenumber (cm )
1680
1530
1490
Wavenumber (cm-1)
Fig. 3 a FTIR spectra for (i) pure PVA-chitosan film with (ii) 10 wt%
NH4Br, (iii) 20 wt% NH4Br, (iv) 30 wt% NH4Br, (v) 40 wt% NH4Br, (vi)
50 wt% NH4Br, (vii) 60 wt% NH4Br, and (viii) pure NH4Br salt in the
region of 2,800–3,600 cm−1. b FTIR spectra for (i) pure PVA-chitosan
film with (ii) 20 wt% NH4Br, (iii) 30 wt% NH4Br, (iv) 40 wt% NH4Br,
and (v) 50 wt% NH4Br in the region of 1,490–1,680 cm−1
Figure 3a shows the FTIR spectra for PVA-chitosanNH4Br complexes in the region of 2,800–3,600 cm−1. The
O–H and C–H bands have shifted to lower wavenumbers with
the addition of NH4Br up to 50 wt% NH4Br, Fig. 3a(ii)–(vi)
which confirms that an interaction has occurred between the
PVA-chitosan blend and NH4Br salt [8]. On addition of
60 wt% NH4Br, υas (NH4+) mode and υs (NH4+) mode appear
at 3,079 and 2,934 cm−1 respectively, as shown in Fig. 3a(vii),
where it can be compared to the spectrum of NH4Br powder,
Fig. 3a(viii). This is due to the increasing salt concentration
which suggests that the symmetry of NH4+ becomes lowered
and reflects the interaction between NH4+ and the polymer
[24]. It can be inferred that the excess salt did not dissociate or
the ions recombine to form neutral ion pairs when the salt
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Ionics (2014) 20:1235–1245
the portion of the semicircle can be observed at high frequencies [27]. The immobile polymer chains which were represented by a capacitor however become polarized in the alternating field [28]. The electrolyte/electrode interface is referred
to as a capacitance. When the capacitance is ideal, it will show
a vertical spike parallel to the real axis [29]. However, the
spike inclined at an angle less than 90° is found due to
roughness of the electrode/electrolyte interface or inhomogeneous distribution of salt in polymer matrix [29–31].
The increase in salt concentration leads to the absence of
semicircle within the set of frequency range suggesting that
only resistive component of the polymer electrolytes exists, as
shown in Fig. 4b [28]. However, the addition of more than
30 wt% salt leads to the appearance of the semicircle at highfrequency areas since the ionic migration and bulk polarization are physically in parallel (Fig. 4c). The Rb for all samples
has been calculated from the low-frequency intercept of the
semicircle or the high-frequency intercept of the spike on the
axis. The increase in salt concentration leads to the decrease in
Rb values as the number of mobile charge carriers increases
[26]. In this work, the Rb values are found to be high for lower
conductivity value. This is due to the recombination of ions
which leads to the decrease in the amorphousness of the
concentration is high [25]. On addition of 20 wt% NH4Br salt,
the carboxamide and amine bands have shifted to 1,633 and
1,517 cm−1 respectively, as shown in Fig. 3b(ii). The bands
are continuously shifted to lower wavenumbers with the addition of NH4Br salt up to 60 wt%. From FTIR studies, it can
be concluded that the shift of the bands are the evidence of the
complexation between the PVA-chitosan blend and NH4Br
salt.
Impedance analysis
Figure 4a–c shows the impedance plots for sample with 20,
30, and 40 wt% NH4Br at room temperature. All the impedance plots show typical behavior being a semicircular portion
at high frequencies and a spike (residual tail) at low frequencies. The semicircle shown in Fig. 4a represented by a parallel
combination of a capacitor due to the immobile polymer chain
and a resistor due to the mobile ions inside the matrix polymer
[26]. This result suggests that the migration of ions may occur
through the free volume of matrix polymer which can be
represented by a resistor. The high-frequency semicircle corresponds to the bulk response of the films. The ionic migration
and bulk polarization are physically in parallel, and therefore,
a
b
5000
600
20
15
4000
450
Zi (ohm)
3000
2000
10
Rb
5
0
300
0
5
10
15
20
150
1000
Rb
0
0
0
1000
2000
3000
4000
0
5000
150
Zr (ohm)
c
300
Zr (ohm)
1000
800
Zi (ohm)
Zi (ohm)
Fig. 4 Impedance plot of PVAchitosan electrolyte with a
20 wt% NH4Br, b 30 wt%
NH4Br, and c 40 wt% NH4Br at
room temperature
600
400
200
Rb
0
0
200
400
600
Zr (ohm)
800
1000
450
600
Ionics (2014) 20:1235–1245
1239
polymer electrolytes which will be shown in XRD
diffractogram.
Conductivity study
The variation of room temperature conductivity as a function
of NH4Br concentration is shown in Fig. 5. The conductivity
increases to (7.68±1.24)×10−4 S cm−1 as the salt concentration increases to 30 wt%. Hema et al. [32] reported that the
highest conductivity of 5.70×10−4 S cm−1 was obtained when
PVA was doped with 25 mol% of NH4Br. The higher conductivity value obtained in this work shows that the ionic conductivity can be increased by using polymer blend as a host. It
is proven that blending two polymers can provide more complexation sites in the polymer electrolyte where ion migration
and exchange can take place. Hence, the ionic conductivity
increases [8]. Further addition of NH4Br leads to the decrease
in conductivity. When more than 30 wt% NH4Br was added,
the ions recombine and form neutral ion pairs since the distance between the dissociated ions become too close [4]. This
leads to the decrease in ionic conductivity.
Buraidah et al. [8] reported that the highest conducting
sample of 55 wt% (chitosan-PVA)-45 wt% NH4I has a conductivity of 1.77×10−6 S cm−1 which is lower compared to the
conductivity value obtained in this work. In their work, the
viscosity of chitosan is ≥400 mPa s. In the present work, the
viscosity is much higher where the range is within 800–
2,000 mPa s. Furthermore, chitosan used in this work has high
molecular weight with degree of deacetylation>75 %. Highly
deacetylated chitosan leads to the increase in viscosity and has
a more flexible chain because of the charge repulsion in the
molecule [12]. PVA used in this work is 80 % hydrolyzed
where the molecular weight is in the range of 9,000–10,000.
1.0E-02
The difference in conductivity values may also be attributed to
the different ratio of PVA-chitosan used in the present work.
The plot of conductivity for the highest conducting sample
as a function of temperature is shown in Fig. 6. The temperature range is from 298 to 373 K. The plot shows that the
conductivity increases with the increase in temperature, which
attributed to the increase in number density and mobility of
ions [4, 33]. The regression value, R2 for the plot is close to 1,
suggesting that the plot is linear and obeyed the Arrhenius
equation:
−E a
σ ¼ σ0 exp
kT
ð2Þ
where σ0 is a pre-exponential factor, Ea is the activation
energy of conduction, and k is the Boltzmann constant. The
Ea value for the highest conducting sample is 0.15 eV, which
was calculated using the slope of the plot and Arrhenius
equation. This result implies that the ions in highly conducting
samples require lower energy for migration [8]. Kadir et al. [4]
reported that the conductivity of 7.90×10−4 S cm−1 obtained
the E a value of 0.15 eV in plasticized PVA-chitosanammonium nitrate (NH4NO3) system. Lower activation energy also resulted from the short distance between transit sites
provided by the blended polymers [8].
XRD analysis
The X-ray diffractograms in Fig. 7 show an increase in broadness with the addition of 0–30 wt% NH4Br which reveals the
amorphous nature of the system. As suggested by Hodge et al.
[34], a complete dissociation of salt in a polymer matrix is
proved when there are no peaks found. The X-ray diffraction
patterns show that the samples are highly amorphous until
30 wt% NH4Br is added to the electrolyte. With the addition of
-2.0
-2.4
Log σ (S cm-1)
Conductivity, σ(S cm-1)
-2.2
1.0E-04
1.0E-06
1.0E-08
-2.6
-2.8
-3.0
-3.2
-3.4
-3.6
-3.8
-4.0
1.0E-10
0
20
40
60
80
NH4Br content (wt.%)
Fig. 5 Effect of NH4Br content on conductivity of electrolyte at room
temperature
2.5
2.7
2.9
3.1
-1
3.3
3.5
-1
1000T (K )
Fig. 6 Conductivity of PVA-chitosan with 30 wt% NH4Br at elevated
temperature
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Ionics (2014) 20:1235–1245
chitosan film and PVA-chitosan doped with 10, 30, and
60 wt% NH4Br are shown in Fig. 8b. As the temperature
increases from 30 to 100 °C, the TGA curves indicate only
∼10 % weight loss which may correspond to the loss of
adsorbed and bound water, and also the residue of acetic acid
[37]. Alias et al. [38] stated that the mass loss from room
temperature to 600 °C corresponds to the evaporation of water
and decomposition of thermally unstable organic matter. The
major weight loss of 40 % in chitosan film which starts at
around 200 °C and continues up to 400 °C is attributed to the
decomposition of chitosan. Decomposition of PVA occurs in
three stages. PVA film decomposes at around 260 °C which
was found to be a 70 % loss in weight, accompanied by the
formation of some volatile products. The greatest weight loss
is believed to be the disintegration of intermolecular and
Intensity (a.u.)
(g)
(f)
(e)
(a)
PVA film
(d)
Weight loss (%)
100
(c)
(b)
(a)
5
20
35
50
65
120
Chitosan
film
Pure PVAchitosan
80
60
40
80
20
2θ (degree)
Fig. 7 X-ray diffraction patterns of (a) 0 wt% NH4Br, (b) 10 wt%
NH4Br, (c) 20 wt% NH4Br, (d) 30 wt% NH4Br, (e) 40 wt% NH4Br, (f)
50 wt% NH4Br, and (g) pure NH4Br salt (at room temperature)
0
0
200
400
600
800
Temperature (˚C)
TGA analysis
TGA has been used as a method to examine the thermal
properties by looking at the decomposition temperatures
[35]. According to Aziz et al. [36], it is necessary for the
polymer electrolytes to have high thermal stability in some
applications, for example, batteries. Therefore, TGA analysis
was carried out where this method can also be used to investigate the percentage of weight loss and phase transitions.
TGA curves for pure PVA film, pure chitosan film and pure
PVA-chitosan film are shown in Fig. 8a while pure PVA-
(b)
120
Pure PVAchitosan
100
Weight loss (%)
40 wt% NH4Br, some crystalline peaks have appeared at 2θ=
21.9°, 31.1°, 38.3°, 44.6°, 50.2°, 55.4°, 69.5° and 73.7°.
These peaks were attributed to the recrystallization of NH4Br
out of the film surface. Intensity of the NH4Br peaks increases
as the salt concentration increases. This is because the polymer host was unable to accommodate the salt which leads to
the recombination of the ions and resulted in conductivity
decrement [4].
10 wt.% NH4Br
80
30 wt.% NH4Br
60
60 wt.% NH4Br
40
20
0
0
200
400
600
800
Temperature (˚C)
Fig. 8 TGA results of a pure PVA film, pure chitosan film and pure PVAchitosan blend film and b pure PVA-chitosan blend film and PVAchitosan with 10, 30, and 60 wt% NH4Br
Ionics (2014) 20:1235–1245
partial breaking of the molecular structure [37]. However, at
800 °C, PVA film shows that only 5 % left compared to
chitosan film where there is still 40 % material left. It was
observed that the degradation behavior of chitosan is retained
in the PVA-chitosan blend. At 800 °C, PVA-chitosan film
shows 20 % material left which proved that blending PVA
with chitosan has improved the stability as shown in Fig. 8b.
The increasing amount of salt has been attributed to the loss of
volatile material and could also attribute to the formation of
other volatile residues.
FESEM study
Figure 9a–c shows the FESEM micrographs of selected samples. It is observed that the micrograph of PVA-chitosan
containing 10 wt% NH4Br, Fig. 9a, shows a smooth and
homogenous surface indicating that the polymer blend and
10 wt% of NH4Br are miscible into each other [37]. The
polymer electrolyte attains the highest conductivity with the
addition of 30 wt% NH4Br. As can be seen in Fig. 9b, the
morphology consists of grains which are uniformly dispersed
on the surface of the film are inferred as ions trapped in the
Fig. 9 FESEM micrograph of
PVA-chitosan blend film containing a 10 wt% NH4Br, b 30 wt%
NH4Br, and c 40 wt% NH4Br
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blended film [4]. Figure 9c shows the morphology which
partially consists of solid structures that have protruded out
of the surface of the film, revealing that the salt has recrystallized out of the film and giving the answer to the decrease in
conductivity when more than 30 wt% of NH4Br is added.
Dielectric study
Dielectric study is useful in revealing the conductivity behavior of polymer electrolyte. This study gives an important
insight into the polarization effect at the electrode/electrolyte
interface and further understanding in conductivity trend [39].
Figure 10a, b shows the frequency dependence of real and
imaginary parts of the dielectric constants. The equations for
the dielectric constant, εr and dielectric loss, εi are as follows:
εr ¼
Zi
ωC 0 Z 2r þ Z 2i
ð3Þ
εi ¼
Zr
ωC 0 Z 2r þ Z 2i
ð4Þ
(a)
(b)
(c)
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Ionics (2014) 20:1235–1245
(a)
1.00
80000
s = -0.0023T + 1.2606
70000
10 wt.% NH4 Br
60000
20 wt.% NH4 Br
0.80
40 wt.% NH4 Br
40000
50 wt.% NH4 Br
0.60
s
εr
30 wt.% NH4 Br
50000
0.40
60 wt.% NH4 Br
30000
20000
0.20
10000
0.00
0
1
2
3
4
5
6
295
7
305
315
335
345
355
365
T (K)
Log f (Hz)
(b)
325
Fig. 12 Plot s versus T for PVA-chitosan blend film with 30 wt% NH4Br
60000
10 wt.% NH4 Br
50000
20 wt.% NH4 Br
30 wt.% NH4 Br
40000
εi
40 wt.% NH4 Br
50 wt.% NH4 Br
30000
60 wt.% NH4 Br
20000
10000
0
1
2
3
4
5
6
7
Log f (Hz)
Fig. 10 Frequency dependence of a εr and b εi at room temperature
where Zr and Zi are the real and imaginary parts of the
impedance, ω is angular frequency and C0 is the vacuum
capacitance.
The dielectric constant is a measure of stored charge. At
low frequencies, both εr and εi rise due to electrode polarization effects [40]. However, the periodic reversal of the electric
field occurred so fast at high frequencies; hence, no excess ion
diffusion was observed in the direction of the field [28]. The
values of εr and εi also decrease as a result of the decrease in
polarization, which conforms to the non-Debye behavior of
the polymer electrolytes. The addition of salt, resulting in an
increase in the number of free ions or charge carriers density,
increases the conductivity [41]. The variations of εr and εi
follow the same trend as the conductivity where PVA-chitosan
with 30 wt% NH4Br obtained the highest values of εr and εi.
The dielectric study for starch-chitosan-NH4NO3 system reported by Khiar and Arof [42] also follows the same trend as
their conductivity result, proving that the enhancement in
number density of charge carriers could attribute to the increasing conductivity.
Conduction mechanism
In general, ac conductivity σac can be calculated from the
equation:
σac ¼ εo εr ωtanδ
ð5Þ
or
σac ¼ εo εi ω
ð6Þ
11.2
11.0
298K
303K
308K
313K
318K
323K
328K
333K
338K
343K
348K
353K
358K
363K
ln εi
10.6
10.4
10.2
10.0
9.8
9.6
9.4
12.5
12.6
12.7
12.8
0.70
0.60
0.50
Current (mA)
10.8
0.40
0.30
0.20
0.10
0.00
-0.10
12.9
ln ω (Hz)
Fig. 11 ln εi versus ln ω at different temperatures for PVA-chitosan
blend film with 30 wt% NH4Br
-0.20
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
Voltage (V)
Fig. 13 LSV curves of PVA-chitosan with 30 wt% NH4Br at room
temperature
Ionics (2014) 20:1235–1245
1243
where εi is dielectric loss and tan δ=εi /εr. Generally, σac is
analyzed using the Jonscher’s universal power law [43, 44]:
σðωÞ ¼ Aωs þ σdc
ð7Þ
where σ(ω) is the total of ac and dc conductivity, A is a
temperature-dependent parameter, s is the power law exponent and σdc is frequency independent dc conductivity. Given
that σac =Aωs, the value of s is obtained from the following
relation:
lnεi ¼ ln
A
þ ðs−1Þlnω
εo
ð8Þ
Figure 11 shows the plot of ln εi against ln ω. The value of
exponent s is obtained from the slope at high-frequency region
where no or minimal electrode polarization occurs [13].
Buraidah et al. [14] and Kadir et al. [45] also suggested that
the acceptable frequency range is at the high-frequency region. In this work, the acceptable frequency range is 12.55≤ln
ω≤12.77.
The plot of s against temperature for the highest conducting
electrolyte sample is shown in Fig. 12. It is observed that s
decreases with increasing temperature. As the exponent s is
temperature-dependent, this implies that the conduction
mechanism of the present system follows the CBH model.
The plot can be best represented by the equation of s=
−0.0023T+1.2606. From the equation, it can be concluded
that s→1 when T→0. In the CBH model, the charge carriers
are assumed to be surrounded by several potentials such as
coulombic repulsive potential between the ions and the potential well [46]. According to Buraidah et al. [14], when the
ions have gained enough energy, they hop from one site to
another where two competing relaxation processes may occur.
These ions may hop back to the initial state or form a new
absolute potential with an increase in back-hop barrier height
and they continue to move in the forward direction. Shukur
et al. [47] reported that the conduction mechanism for starchlithium iodide (LiI) electrolyte occurs by way of CBH model
where the plot of exponent s against T can be fitted to the
equation s=−0.0023T+0.9297 which is comparable with this
work. These hops are thermally activated where the s value
decreases with increasing temperature up to 353 K.
LSV study
Figure 13 depicts a linear sweep voltammogram for the
highest conducting electrolyte. The electrochemical stability
is an important parameter for the characterization of prepared
polymer electrolyte. To test the performance of any device, the
electrochemical stability of the sample must be known prior to
charge-discharge cycling test. The breakdown voltage is important to prevent the electrolyte from being decomposed
[48]. From the figure, it can be observed that the decomposition voltage of the sample is 1.57 V at room temperature.
Kadir and Arof [49] reported that the voltage breakdown for
PVA-chitosan-based membrane containing NH4NO3 and ethylene carbonate plasticizer is ∼1.70 V at room temperature.
Thus, this sample can be used as solid polymer electrolyte in
the fabrication of protonic batteries since the standard electrochemical window of protonic batteries is about ∼1.0 V [50].
Conclusion
From FTIR studies, it is proven that complexation has occurred between the polymer blend host and the salt. The
highest room temperature conductivity is obtained at (7.68±
1.24)×10−4 S cm−1 with addition of 30 wt% NH4Br, where
the Ea value is 0.15 eV. XRD result shows that PVA-chitosan
with 30 wt% NH4Br exhibits the most amorphous polymer
electrolyte. TGA result indicates that the thermal stability of
PVA has been improved by the addition of chitosan. From
FESEM analysis, it can be concluded that the variation of
conductivity is influenced by the morphology of the samples.
The trend of εr and εi as a function of salt composition
suggests that the increase in conductivity is mainly due to
the increase of the number of charge carriers. Dielectric study
suggests that the samples show non-Debye behavior. The
conduction mechanism for the highest conducting electrolyte
is best represented by the CBH model when exponent s→1 as
T→0. LSV has shown that the polymer electrolyte can be
used as an electrolyte in batteries application.
Acknowledgments The authors would like to thank the University of
Malaya for the financial support provided (Grant No. PG050-2012B).
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