J Solid State Electrochem
DOI 10.1007/s10008-014-2697-3
ORIGINAL PAPER
Electrical conductivity characterization
of polyacrylonitrile-ammonium bromide polymer
electrolyte system
S. Sikkanthar & S. Karthikeyan & S. Selvasekarapandian &
D. Vinoth Pandi & S. Nithya & C. Sanjeeviraja
Received: 21 September 2014 / Revised: 26 November 2014 / Accepted: 1 December 2014
# Springer-Verlag Berlin Heidelberg 2014
Abstract A new proton-conducting polymer electrolyte
based on polyacrylonitrile (PAN) doped with ammonium
bromide (NH4Br) has been prepared using solution casting
technique. The complexation of NH4Br with PAN polymer
has been studied using X-ray diffraction (XRD) and Fourier
transform infrared spectroscopy (FTIR). The differential scanning calorimetry (DSC) thermograms of PAN with NH4Br
electrolyte membrane show the decrease in glass transition
temperature (Tg). This reduction of Tg of membrane reveals
the increase of segmental motion of polymer electrolyte. The
ionic conductivity of the prepared polymer electrolyte has
been found by ac impedance spectroscopic analysis. The
maximum ionic conductivity (2.5×10−3 S cm−1) has been
obtained for 30 mol% NH4Br-doped PAN polymer electrolyte. The temperature-dependent conductivity of the polymer
electrolyte follows an Arrhenius relationship. The dielectric
spectra show low frequency dispersion. The relaxation time
S. Sikkanthar
Research and Development Centre, Bharathiar University,
Coimbatore, Tamilnadu, India
S. Sikkanthar
PG & Research Department of Physics, Arignar Anna Government
Arts College, Cheyyar, Tamilnadu, India
S. Sikkanthar : S. Selvasekarapandian (*) : D. V. Pandi
Materials Research Centre, Coimbatore, Tamilnadu, India
e-mail: [email protected]
S. Karthikeyan
Department of Physics, Madras Christian College, Tambaram,
Tamilnadu, India
S. Nithya
Department of Physics, SRNM College, Sattur, Tamilnadu, India
C. Sanjeeviraja
Department of Physics, Alagappa College of Engineering &
Technology, Karaikudi, Tamilnadu, India
(τ) has been calculated from loss tangent spectra (tan δ). Ionic
transference number measured has been found to be in the
range of 0.92–0.99 for all the polymer electrolyte system. The
result reveals that the conducting species are predominantly
ions. Using the maximum ionic conducting polymer electrolyte, the primary proton-conducting battery with configuration
Zn+ZnSo4·7H2O/70 PAN:30 NH4Br/PbO2 + V2O5 has been
fabricated, and its discharge characteristics have been studied.
Keywords Proton conduction . XRD . FTIR . DSC .
Impedance spectroscopy . Dielectric spectroscopy
Introduction
Solid proton-conducting polymer electrolytes have been given
more interest due to their variety of applications in electrochemical devices such as batteries, chemical sensor supercapacitors,
fuel cells, and electrochromic displays [1]. The pioneering work
of Armand et al. [2] and others [3, 4] led to development of
polymer-based electrolytes for battery applications. Many polymer electrolytes have been developed based on polymer polyethylene oxide (PEO), polyvinyl alcohol (PVA), polymethyl
methacrylate (PMMA), poly(vinylidene fluoride-hexafluoro
pylene) (P(VDF-HFP)), poly(acrylo nitrile-methyl methacrylate) (P(AN-MMA)), and poly(acrylo nitrile-co-methyl methacrylate-co-styrene) (PAMS) [5–8]. Proton-conducting polymer
complexes with inorganic acids have been shown to suffer from
chemical degradation and mechanical integrity making them
unsuitable for practical applications. Considering this fact and
due to lack of good proton-conducting polymer electrolyte
working at ambient temperatures, search for the new systems
has been perused in the past few years.
Characterization of PVA-NH4NO3 polymer electrolyte and
its application in rechargeable proton battery have been reported by Selvasekarapandian et al. [9]. Majid et al. have been
J Solid State Electrochem
studied proton-conducting polymer electrolyte films based on
chitosan acetate complexed with NH4NO3 [10]. Plasticized
PEO + NH4PF6 proton-conducting polymer electrolyte system has been studied by Kuldeep and Mishra [11]. The polymer electrolytes based on PVAc/PMMA and PVAc/PVdF has
been reported by Baskaran et al. [12]. Hirankumar et al. [13]
have been studying proton-conducting polymer electrolyte
complexes based on PVA + CH3COONH4. Using protonconducting k-carrageenan-chitosan electrolytes for electrical
double layer capacitor has been done by Shuhaimi et al. [14].
Hema et al. [15] reported the proton-conducting polymer
electrolyte based on PVA doped with NH4NO3. The protonconducting polymer electrolyte based on PVP with NH4SCN
has been studied by Ramya et al. [16].
Polyacrylonitrile (PAN) polymer has been extensively
studied due to their good chemical and flame resistance and
electrochemical stability [17]. PAN is a semicrystalline, synthetic resin prepared by the polymerization of acrylonitrile. As
a member of the important family of acrylic resins, it is hard,
rigid, slow to burn, and of low permeability to gases. PAN is
commonly used in home furnishers in the place of wool in
fiber form. Fibers of polyacrylonitrile have been used in hot
gas filtration systems also.
El-Sobati et al. [18] studied 2,2-azo-bisisobutyronitrile
(AIBN) initiated polymerization of acrylonitrile–metal complex salt in dimethylformamide (DMF) with transition metals
(as a catalyst) and obtained a PAN–metal complex salt. Wu
et al. [19] studied heated PAN containing the copper(I) ion and
confirmed the complex structure between PAN and copper(I)
ion. The effect of heat treatment on the dielectric relaxation of
PAN was studied by Gupta et al. [20]. Ishikawa et al. [21]
reported the use of polymer gel electrolytes based on PAN and
propylene carbonate (PC) for capacitor applications. PANbased gel polymer electrolytes with lithium and magnesium
triflate ionic salts are used to fabricate electrochemical double
layer capacitors in conjunction with EC and PC as plasticizers
and high-density graphite (HDG) as electrode by Mitra et al.
[22]. PAN-EC-PC-lithium trifluromethane sulfonate
(LICF3SO3) polymer electrolytes have been studied for rechargeable lithium batteries by Perera et al. [23].
There is no work on PAN with ammonium salt-based proton-conducting polymer electrolyte except by Nithya et al. [24].
In the present study, PAN doped with different molar concentrations of ammonium bromide (NH4Br) has been prepared by
solution casting method. Ammonium salts are very good proton donor for developing membrane with highest proton conductivity [25]. The prepared polymer electrolytes have been
characterized by X-ray diffraction (XRD), Fourier transform
infrared spectroscopy (FTIR), and electrical conductivity studies. Differential scanning calorimetry (DSC) aims to study the
thermal stability of polymer electrolytes. The ionic transference
number measurement of polymer electrolyte system has been
estimated by Wagner’s dc polarization method.
Experimental studies
PAN (MW=150,000, Sigma-Aldrich) and NH4Br (AR grade,
Merck) have been used as the raw materials in this study.
DMF is used as a solvent. Appropriate quantities of PAN and
NH4Br in DMF are stirred continuously for several hours until
a homogenous solution has been obtained. The obtained solution is then casted in petri dishes and is subjected to drying at
60 °C for 2 days in a vacuum chamber. Mechanically strong,
transparent, and flexible films of thickness ranging from 0.2 to
0.4 μm are obtained. X-ray diffraction patterns have been
recorded using Philips X-ray diffractometer PW 1830 with
Cu target (λ=154,060 Å). FTIR measurements have been
made with PerkinElmer 57716 R spectrometer in the range
of 400–4,000 cm−1. The glass transition temperature of polymer electrolytes have been obtained from DSC measurements
using a differential scanning calorimeter model DSC-6100
(Seiko Instruments Inc.) at temperature range of 25–300 °C
with the heating rate of 5 °C/min. Conductivity studies of
polymer electrolytes have been carried out in the temperature
range of 303–343 K over a frequency range from 42 to 1 MHz
performed on HIOKI 3532 LCR meter using aluminum as the
blocking electrode. The transference number measurements
have been calculated using Wagner’s dc polarization technique for all electrolyte systems.
Results and discussion
X-ray diffraction analysis
Figure 1a–g represents the XRD pattern of pure PAN, 5, 10,
15, 20, 25, and 30 mol % of NH4Br doped with PAN. A broad
peak around 17° has been observed in the XRD pattern of pure
PAN. The characteristic peak at 17° corresponds to orthorhombic PAN (110) reflection [26]. In the salt-added system,
the peak (17°) has been found to increase in broadness and
decrease in intensity [27]. The decrease of intensity and the
increase in full width half maximum of the characteristics
peak reveal the amorphous nature of the polymer electrolyte.
This result can be interpreted by Hodge et al. criterion which
establishes a correlation between the intensity of the peak and
the degree of crystallinity [28]. This amorphous nature increases as the salt content is increased up to 15 mol% as
observed from Fig. 1a–d. No peaks corresponding to NH4Br
have been observed which indicate a complete dissociation of
salt in the polymer matrix. Figure 1e–g shows that the polymer
has become completely amorphous, and peaks at 30.759,
37.838, 44.056, 49.528, and 54.638 correspond to the dopant
salt (JCPDS card number: 85-099). It shows incomplete dissociation of the salt in the polymer matrix. These peaks
attributed to the recrystallization of NH4Br out of the film
surface. Intensity of the NH4Br peaks increases as the salt
J Solid State Electrochem
concentration increases from 20 to 30 mol%. This is because
the polymer host was unable to accommodate the salt which
leads to the recombination of the ions [29].
FTIR spectroscopic analysis
On addition of salt into the polymer host, the cation of the salt
is expected to coordinate with the polar groups in the host
polymer matrix resulting in the complexation. This type of
interaction influences the local structure of the polymer backbone, and certain infrared active modes of vibration will get
affected. This indicates that the infrared spectroscopic studies
provide the evidence of the complexation. Figure 2 shows the
FTIR spectra of pure PAN and PAN doped with different
concentrations of NH4Br. The band at 869 cm−1 is assigned
to CH rocking of pure PAN. The absorption peak at
1,252 cm−1 has been assigned to CH wagging of pure PAN.
The absorption peak at 1,361 cm−1 has been assigned to CH
partial bending of pure PAN. The CH bending in CH2 is
assigned at 1,450 cm−1 in pure PAN. The intensity of the
peaks (876, 1,252, 1,361, and 1,450 cm−1) has been reduced
due to the addition of NH4Br. The absorption peak at
1,663 cm−1 has been assigned to C=C stretching in pure
PAN. The intensity has been reduced for 5 mol% of NH4Brdoped PAN. The intensity of above-mentioned peaks has
increased on addition of 15–35 mol% NH4Br-doped PAN,
and the peak has become sharp. The intensity of the band at
2,243 cm−1 assigned to the stretching vibration of nitrile
(C≡N) has been reduced 90 % compared to undoped PAN
due to the addition of NH4Br from 5 to 35 mol%. It is the most
characteristic band of pure PAN [30, 31]. The absorption peak
at 2,928 cm−1 has been assigned to symmetric stretching of
NH4+. The intensity of the band at 2,928 cm−1 has been
reduced due to the addition of NH4Br from 5 to 35 mol%.
The bands at 3,466 cm −1 are assigned to asymmetric
stretching modes of NH4+ groups. It is observed from the
spectrum that there is no appreciable change in spectral position of NH4Br-doped PAN with that of pure PAN. However, it
is noted that there is an appreciable change in the intensity of
the various peaks of NH4Br-doped PAN compared to pure
PAN. This shows that the complexation has occurred between
PAN and ammonium bromide. Vibrational peaks and assignments of Pure PAN, 5, 15, 20, 25, 30 and 35 mol% of NH4Br
doped with PAN are tabulated in Table 1.
The possible interaction between the polymer and the
doping salt has been shown in Scheme 1. In general, the
proton conduction can occur either by the lone proton migration mechanism [32, 33] or the proton carried migration
mechanism (vehicular mechanism). However, in the present
case, lone proton migration (H+) mechanism is more probable
because of the following reason. In ammonium bromide, the
hydrogen bonding occurs with the N–H bond with in the
tetrahedral ion, NH4+, pointing directly toward the bromine
Fig. 1 XRD pattern for (a) pure PAN, (b) 95 PAN:05 NH4Br, (c) 90
PAN:10 NH4Br, (d) 85 PAN:15 NH4Br, (e) 80 PAN:20 NH4Br, (f) 75
PAN:25 NH4Br, and (g) 70 PAN:30 NH4Br
ion Br− and forming an N–H…..Br hydrogen bond [34].
Three of the four hydrogens of NH4+ ions are bounded strongly with nitrogen ion identically and the fourth is weakly
bound. The weakly bound H of NH4+ can easily be dissociated
under the influence of an electric field. These H+ ions can hop
via each coordinating site (C≡N) of the host polymer (PAN)
and thus conduction takes place. Hence, from FTIR spectroscopy, the interaction between PAN and NH4Br has been
confirmed.
Fig. 2 FTIR spectra for (a) pure PAN, (b) 95 PAN:05 NH4Br, (c) 85
PAN:15 NH4Br, (d) 80 PAN:20 NH4Br, (e) 75 PAN:25 NH4Br, (f) 70
PAN:30 NH4Br, and (g) 65 PAN:35 NH4Br
J Solid State Electrochem
Differential scanning calorimetric studies
The thermal analysis has been performed using differential
scanning calorimetric studies. The changes in the glass transition temperatures of PAN by the addition of NH4Br salt are
observed. Figure 3a, b represents the DSC plot for 80–90 and
100–400 °C, respectively. Figure 3a–f shows DSC thermogram of pure PAN, 95 PAN:05 NH4Br, 90 PAN:10 NH4Br, 75
PAN:25 NH4Br, 70 PAN:30 NH4Br and 65 PAN:35 NH4Br
respectively. The Tg values of polymer electrolyte with different compositions are tabulated in Table 2. The glass transition
temperature of pure PAN is 88.1 °C reported by Dissanayake
et al. [35]. It has been observed that the Tg of polymer
electrolytes decreases with the increase of the concentrations
of NH4Br salt up to 30 mol%. The decrease in Tg indicates the
increase in mobility of polymer chains [36]. This may be due
to plasticization effect of the electrolyte with the addition of
salt which enhances the proton transport. The complexation of
70 PAN:30 NH4Br has been found to have a low Tg of
82.3 °C. The low glass transition temperature causes the
higher segmental motion of the polymer electrolyte [37].
However, for greater concentrations, the Tg has been found
to increase which may be due to the presence of some undissociated salt in the host polymer matrices. This observed shift
in Tg values of the polymer electrolytes in DSC thermograms
indicates the interaction between the polymer and the salt.
Similar results have also been previously reported low glass
transition temperature Tg for 25 mol% NH4Br-doped PVA by
Hema et al. The low values of Tg represent the highly amorphous nature of the polymer electrolyte as confirmed by XRD
results [38]. This is suggestive of enhanced segmental motions, resulting in higher conductivity.
Scheme 1 Possible interaction between the host polymer (PAN) and the
dopant (NH4Br)
In this case, the disappearance of semicircular portion in
the impedance curve leads to a conclusion that the current
carriers are ions, and this leads one to further conclude that the
total conductivity is mainly the result of ion conduction [40].
The fitting of the complex impedance plot by using the “EQ”
developed by Boukamp [41, 42] yields the bulk resistance
Electrochemical impedance spectroscopy analysis
Electrochemical impedance spectroscopy is a powerful method
of characterizing many of the electrical properties of electrolyte
materials and their interfaces with electronically conducting
electrodes. Figure 4a shows the impedance plot for different
mole ratios of PAN doped with NH4Br polymer electrolyte
system at room temperature (303 K). Normally, the complex
impedance plot consists of a high frequency depressed semicircle represented by a frequency dependent capacitor (Cg)
parallel to a bulk resistor (Rb) and a low frequency spike
represented by a constant phase element. The migration of ions
may occur through the free volume of polymer matrix, which
can be represented by a resistor. The immobile polymer chains
become polarized in the alternating field, which can be represented by a capacitor. The ionic migration and bulk polarization
are physically in parallel and therefore gives a semicircle. The
inclined straight line at the low frequency region could be the
effect of electrode and electrolyte interface [39].
Fig. 3 a DSC plot for (a) pure PAN, (b) 95 PAN:05 NH4Br, (c) 90
PAN:10 NH4Br, (d) 75 PAN:25 NH4Br, (e) 70 PAN:30 NH4Br, (f) 65
PAN:35 NH4Br for temperature range (80–90 °C). b DSC plot for (a)
pure PAN, (b) 95 PAN:05 NH4Br, (c) 90 PAN:10 NH4Br, (d) 75 PAN:25
NH4Br, (e) 70 PAN:30 NH4Br, and (f) 65 PAN:35 NH4Br for temperature
range (90–400 °C)
J Solid State Electrochem
Table 1 Vibrational peaks and assignments of pure PAN, 95 PAN:05 NH4Br, 85 PAN:15 NH4Br, 80 PAN:20 NH4Br, 75 PAN:25 NH4Br, 70 PAN:30
NH4Br, and 65 PAN:35 NH4Br polymer electrolytes
Pure PAN
2,243
1,663
1,450
1,361
1,252
869
95 PAN:05
NH4Br
85 PAN:15
NH4Br
80 PAN:20
NH4Br
75 PAN:25
NH4Br
70 PAN:30
NH4Br
65 PAN:35
NH4Br
Assignment
3,441
2,929
2,243
1,659
1,450
1,386
1,252
876
3,402
2,928
2,243
1,651
1,447
1,389
1,252
3,401
2,928
2,243
1,650
1,447
1,389
1,250
3,400
2,930
2,243
1,650
1,446
1,389
1,251
822
3,401
2,930
2,243
1,650
1,446
1,389
1,251
3,401
2,931
2,243
1,650
1,446
1,389
1,251
NH4+ asymmetric stretching
NH4+ symmetric stretching
C≡N stretching
C=C stretching
CH bending of CH2
CH plane bending
CH wagging
CH rocking
(Rb). The equivalent electrical circuit is shown in Scheme 2.
Electrochemical impedance spectroscopy (EIS) parameters have
been extracted from Cole–Cole plot. In Table 3, the resistance of
pure PAN is 324. The value of resistance has been decreased
from 254 to 98 Ω for 5–35 mol% of NH4Br doped with PAN
polymer electrolyte. The constant phase element (CPE) value of
pure PAN is 146 μF. The CPE value of NH4Br doped with PAN
polymer electrolyte is in the range of 64–109 μF. The n value of
pure PAN is 0.42. The n value of NH4Br doped with PAN
polymer electrolyte is in the range of 0.579–0.83. The highest
conductivity polymer electrolyte (70 PAN:30 NH4Br) is R=
118 Ω, CPE=72 μF, and n=0.83. In similar studies, EIS parameters of the carbonized PAN fibers have been reported by Li et al.
[43]. Ramanavicius et al. have studied the EIS parameters of PD/
glucose oxide-modified graphite electrode [44].
The bulk resistance (Rb) of the polymer electrolytes has
been extracted from the low frequency intercept on the Z′ axis.
The ionic conductivity is calculated using the equation
σ¼
l
S cm‐1
ARb
ð1Þ
where l is the thickness of the polymer electrolyte and A is
the contact area. It has been found that 30 mol% of NH4Br
doped with PAN has the highest conductivity of 2.5 ×
10−3 S cm−1 at 303 K. Inspite of the appearance of the peaks
due to NH4Br, the conductivity is maximum because of its
complete amorphous nature.
Figure 4b represents the impedance plot for the highest
conductivity sample (70 PAN:30 NH4Br) at different temperatures. The semicircle disappears indicating the prevalence of
the resistive component of the electrolyte system [45]. The
conductivity increases with the increase of temperature which
Fig. 4 a Cole–Cole plot for 95 PAN:05 NH4Br, 90 PAN:10 NH4Br, 85
PAN:15 NH4Br, 80 PAN:20 NH4Br, 75 PAN:25 NH4Br, 70 PAN:30
NH4Br, and 65 PAN:35 NH4Br at 303 K. b Cole–Cole plot for 70
PAN:30 NH4Br polymer electrolyte at different temperature
R
Scheme 2 Equivalent circuit
CPE
J Solid State Electrochem
conductivity sample 30 mol % NH4Br doped with PAN at
different temperatures, respectively. The conductance spectra
consist of three distinct regions. The low frequency dispersion
region can be ascribed to the space charge polarization at the
blocking electrode [39]. The final high frequency region conductivity dispersion has been observed, and it is predominant
at low temperature.
The high frequency region for different temperatures has
been explained through Jonschen’s universal power feature
[47].
σðωÞ ¼ σdc þ Aωα
ð2Þ
σdc is the frequency-independent conductivity of the prepared polymer electrolyte. A is the temperature-dependent
dispersion parameter, and α is the power law exponent
(0<α< 1) that has been fitted to experimental data at
medium and high frequency region using the non-linear
least square fitting procedure. From the result, it has
been found that the dc conductivity values are in good
agreement with these obtained from the Cole–Cole plot.
From Fig. 5b, it has been found that the dc conductivity
increases with an increase of temperature which suggests
that the free volume around the polymer chain causes the
increase in mobility of ions and polymer segments, and
hence the conductivity increases [48].
Temperature-dependent conductivity
Fig. 5 a Conductance spectra for (a) 5 mol%, (b) 10 mol%, (c) 15 mol%,
(d) 20 mol%, (e) 25 mol%, (f) 30 mol%, and (g) 35 mol% of NH4Br
doped with PAN polymer electrolyte. b Conductance spectra for 30 mol%
NH4Br doped with PAN polymer electrolyte at different temperatures
Figure 6 shows the temperature dependence of ionic conductivity for the various compositions of PAN-NH4Br
polymer electrolytes. It has been observed that the proton
conductivity of the electrolytes increases with increasing
temperature for all complexes. The linear variation of ionic
conductivity with inverse of absolute temperature reveals
can be easily understood on the basis of the ionic transport
mechanism of solid polymer electrolyte. When the temperature is increased, the ionic mobility of the polymer chain is
enhanced and the fraction of free volume in a solid polymer
electrolyte increases accordingly which leads to an increase in
the ionic conductivity of polymer electrolyte [46]. In addition,
the mobility of charge carriers increases with increase of
temperature resulting in an increase in the ionic conductivity
at higher temperatures. Table 2 lists the ionic conductivity
values for the prepared film at different temperatures.
Conductance spectra analysis
Figure 5a, b shows the typical conductance plot for different
mol% of NH4Br doped with PAN at 303 K and for the highest
Fig. 6 Temperature dependence of ionic conductivity for 15, 20, and
30 mol% concentrations of PAN:NH4Br electrolyte
J Solid State Electrochem
Table 2 Conductivity data and
activation energy value for PAN:
NH4Br polymer electrolyte for
different temperatures
PAN:NH4Br composition
σ303 K
σ313 K
σ323 K
Ea (eV)
Regression value
Tg (°C)
95:5
90:10
85:15
80:20
75:25
1.3×10−4
3.6×10−4
6.2×10−4
7.5×10−4
1.2×10−3
1.8×10−4
9.4×10−4
1.3×10−3
2.0×10−3
2.05×10−3
2.2×10−4
3.9×10−4
4.7×10−4
1.4×10-3
3.8×10−3
0.64
0.49
0.45
0.38
0.32
0.99
0.99
0.98
0.95
0.93
84.5
84.0
84.0
83.8
83.7
70:30
65:35
2.5×10−3
3.0×10−4
3.5×10−3
4.8×10−4
4.5×10−3
5.1×10−4
0.28
0.41
0.92
0.97
82.3
85.3
the Arrhenius type thermally activated process given by the
relation
σT ¼ σ0 eð kT Þ
−Ea
ð3Þ
Fig. 7 a Frequency dependence of ε′ (ω) for 95 PAN:05 NH4Br, 90
PAN:10 NH4Br, 85 PAN:15 NH4Br, 80 PAN:20 NH4Br, 75 PAN:25
NH 4 Br, 70 PAN:30 NH 4 Br, and 65 PAN:35 NH 4 Br at room
temperature. b Frequency dependence of ε′ (ω) for 30 mol% of NH4Br
doped with PAN at different temperatures
where σ0 is the pre-exponential factor, Ea is the activation energy, T is absolute temperature, and k is the
Boltzmann constant. Druger et al. [49, 50] have attributed the increase in conductivity with temperature in solid
Fig. 8 a Frequency dependence of ε″(ω) for 95 PAN:05 NH4Br, 90
PAN:10 NH4Br, 85 PAN:15 NH4Br, 80 PAN:20 NH4Br, 75 PAN:25
NH4Br, 70 PAN:30 NH4Br, and 65 PAN:35 NH4Br at 303 K. b
Frequency dependence of ε″(ω) for 30 mol% of NH4Br doped with
PAN at different temperatures
J Solid State Electrochem
Fig. 9 Frequency dependence of M′(ω) for 70 PAN:30 NH4Br at
different temperatures
polymer electrolyte to segmental motion, which results in
an increase in the free volume of system. Thus, the
segmental motion either permits the ions to hop from
one site to another or provides a pathway for ions to
move. The segmental movement of the polymer facilitates the translational motion/hopping facilitated by the
dynamic segmental motion of the polymer. As the amorphous region increases, the polymer chain acquires faster
internal modes in which bond rotation produces segmental motion to favor inter- and intra-chain ion hopping,
and thus the conductivity becomes high.
The activation energy, Ea (combination of the energy of
defect formation and the energy of migration), is calculated
for all the prepared polymer electrolytes by linear fit of the
Arrhenius plot. The calculated activation energy values have
been listed in Table 2. The activation energy decreases with an
increase in salt concentration; this is due to the amorphous
nature of the polymer electrolyte that facilitates the ionic motion in the polymer. The activation energy is low (0.28 eV) for
30 mol % NH4Br doped with PAN polymer electrolyte.
Dielectric spectra analysis
The dielectric behavior of the polymer electrolyte brings about
important insights into ionic transport phenomenon [51]. The
measured impedance data were used to calculate the real and
imaginary parts of the complex permittivity using the relation
0 σ
ε* ¼ ε0 − jε″ ¼ ε0 −j
ð4Þ
ωε0
Real ε′ and imaginary ε″ components are the storage and loss
of energy in each cycle of the applied electric field [52]. σ′ is the
real part of conductivity, ω is the angular frequency, and ε0 is the
Fig. 10 a Frequency dependence of M″(ω) for 5, 10, 15, 20, 25, 30, and
35 mol% of NH4Br doped with PAN polymer at room temperature. b
Frequency dependence of M″(ω) for 70 PAN:30 NH4Br at different
temperatures
permittivity of free space. Figure 7a, b represents frequency
dependence of ε′ (ω) for room temperature and 30 mol% of
NH4Br doped with PAN at different temperatures. In Fig. 7a, the
observed variation in ε′ with frequency could be attributed to the
formation of a space charge region at the electrode and electrolyte interface, which is familiarly known as the non-Debey type
of behavior, where the space charge regions with respect to the
frequency are explained in terms of ion diffusion [53]. The
increase in the dielectric constant represents a fractional increase
in charges within the polymer electrolyte. It is clear that the
values of ε′ (ω) are very high at a very low frequency region. It is
due to the presence of space charge effects which is contributed
by the accumulation of charge carriers near the electrode [54,
55]. At high frequencies, ε′ (ω) have been found to be relatively
constant with frequency. This is because periodic reversal of the
J Solid State Electrochem
Fig. 11 Tangent loss spectra of 95 PAN:05 NH4Br, 90 PAN:10 NH4Br,
85 PAN:15 NH4Br, 75 PAN:25 NH4Br, 70 PAN:30 NH4Br, and 65
PAN:35 NH4Br polymer electrolyte at 303 K
field takes place so rapidly that the charge carriers will be able to
orient themselves in the field direction resulting in a decrease of
dielectric constant [56]. A high dielectric constant has been
found for 30 mol % NH4Br-doped polymer electrolyte.
Figure 8a, b represents frequency dependence of ε″(ω) for room
temperature and 30 mol% of NH4Br doped with PAN at different temperatures. An increase in the value of dielectric constant
and dielectric loss has been observed at higher temperatures and
is attributed to the higher charge carrier density. As temperature
increases, the degree of salt dissociation and redissociation of
ion aggregates increases, resulting in an increase in the number
of free ions or charge carrier density. These high values are due
to the free charges build up at the interface between the material
and the electrodes. For very low frequencies, there is particular
time for charges to build up at the interface before the field
changes the direction, and this contributes to very large apparent
values of ε″(ω). This phenomenon leads to the so-called conductivity relaxation [57, 58].
Fig. 12 Polarization current vs time plot for 70 PAN:30 NH4Br polymer
electrolyte at 303 K
Modulus spectra analysis
Figure 9 represents the frequency dependence of M′(ω) for
different temperatures. Figure 10a, b represents the frequency
dependence of M′(ω) and M″(ω) for room temperature and
30 mol% of NH4Br doped with PAN polymer, respectively.
Both plots show an increase at the high frequency end, but
well-defined dispersion peaks are not observed. This increase
of modulus value in the plot at higher frequencies may be due
to the bulk effect. With the increase of temperature, the height
of the peak decreases suggesting a plurality of relaxation
mechanism [59]. At lower frequencies, it is observed that the
value of M′(ω) and M″(ω) is in the vicinity of zero, indicating
that the contribution of electrode polarization is negligible.
The presence of long tail at the low frequency region also
provides evidences of the large capacitance associated with
the electrode [45].
Loss tangent spectra
Table 3
EIS parameters for all polymer electrolytes
Pure PAN
95 PAN:5 NH4Br
90 PAN:10 NH4Br
85 PAN:15 NH4Br
80 PAN:20 NH4Br
75 PAN:25 NH4Br
70 PAN:30 NH4Br
65 PAN:35 NH4Br
R, Ω
CPE, μF
n
324
254
283
226
169
130
118
98
146
92
66
56
109
85
72
96
0.42
0.66
0.68
0.65
0.67
0.72
0.83
0.579
The dielectric relaxation parameters of the polymer electrolytes can be obtained from dielectric loss tangent (tan δ)
spectrum analysis. The dielectric loss tangent can be defined
by the relation
tan δ ¼
ε ″ ð ωÞ
ε 0 ð ωÞ
ð5Þ
where ε′’ is the dielectric loss factor and ε′ is the dielectric
constant of the dielectric permittivity. The dependence of the
dielectric loss tangent (tan δ) on frequency under different
mol% of NH4Br is shown in Fig. 11. From the figure, it is
J Solid State Electrochem
Table 4 The relaxation
time (τ) for all polymer
electrolytes
Composition
τ (10−03 s)
95 PAN:05 NH4Br
90 PAN:10 NH4Br
85 PAN:15 NH4Br
75 PAN:25 NH4Br
70 PAN:30 NH4Br
31.84
31.05
30.08
30.02
29.05
65 PAN:35 NH4Br
31.85
clearly shown that the tan δ value increases with increasing
frequency and salt concentration at room temperature. It
passes through a maximum value and thereafter decreases.
The absorption peak is described by the relation ωτ=1 where τ
is the relaxation time of hopping process, and ω is the angular
frequency of the external field ω=2πfmax, τ ¼ ω1 . The values
have been calculated and tabulated in Table 4.
The peak maximum shift towards high frequency with
increasing salt concentration which indicates that the jumping
probability per unit time increases with increasing concentration. The dispersion observed at low frequencies could be
attributed to the interfacial polarization mechanism. The
height of the peak increases with increasing concentration
which is due to the increment in the number of charge carriers
for conduction [60]. The low relaxation time has been observed for the high conductivity composition (70 PAN:30
NH4Br) polymer electrolyte.
Transference number measurements
Transference number is a dimensionless parameter which
informs about the contribution of the particular charged species present in the electrolyte (ions and electrons) to overall
charge transport across the cell. When a voltage below the
decomposition potential of the electrolyte is applied to the
cell, migration of the ion will occur until steady state is
reached. At the steady state, the cell is polarized and residual
current flows because of electron migration across the
Fig. 13 Battery holder
electrolyte interfaces. This is because the ionic current through
an ion blocking electrode decreases rapidly with time if the
electrolyte is primarily ionic. The Ii decreases with time due to
the depletion of the ionic species in the electrolyte and becomes constant in the depleted situation [61].
Total ionic transference number of composite polymer electrolytes has been measured using Wagner’s polarization technique [62]. This technique is used to determine the ionic
contribution to the total charge transport by measuring the
residual electronic current passing through the electrolytes.
The cell stainless steel/70 PAN:30 NH4 Br/stainless steel has
been prepared and polarized by applying fixed dc bias voltage
(1.5 V) across the cell. The polarization current passing through
the cell is monitored as a function of time. The result of dc
polarization measurements on the 30 mol% NH4Br electrolyte
(at 303 K) are shown in Fig. 12. The transference numbers are
calculated using the relation
I i −I f
tþ ¼
ð6Þ
Ii
t − ¼ I i −I f
ð7Þ
where Ii is the initial current and If is the final residual
current. The ionic transference numbers (t+) for all compositions of the PAN:NH4Br polymer electrolyte systems are
found to be in the range of 0.92–0.99. This suggests that the
charge transport in these polymer electrolyte systems is predominantly ionic accompanied by mass transport, and electronic contribution to the total current is negligible. The diffusion coefficients of cations and anions of each polymer
electrolytes have been calculated from the measured values
of conductivity and cation transference number, t+, using the
Fig. 14 Open circuit voltage as a function of time for 70 PAN:30 NH4Br
polymer electrolyte
J Solid State Electrochem
Table 6
Important cell parameters
Cell parameter
Cell area (cm2)
Cell weight (g)
Effective cell diameter (cm)
Cell thickness (cm)
Open circuit voltage (OCV) (V)
Discharge time for plateau region (h)
k–
D+
D–
T–
n–
Fig. 15 Discharge curves for cell using 1 MΩ for 70 PAN:30 NH4Br
polymer electrolyte
following equations [63]:
KT σ
Dþ þ D− ¼
ne2
tþ ¼
ð8Þ
Dþ
ðDþ −D− Þ
σ
ne
ð10Þ
ð11Þ
Where,
e–
μ+
μ−
The charge of the electron
The ionic mobility of cation
The ionic mobility of anion
Table 5
Table 5 shows that the cation mobility μ+ has greater value
than the ionic mobility of anions μ−. When μ+ decreases, the
conductivity also decreases and vice versa. The same behavior
also can be detected for D+. Hence, the study of transference
number measurements leads to the conclusion that the conductivity has been influenced by the μ+ and D+.
Fabrication and characterization of primary proton battery
μþ
μþ þ μ−
tþ ¼
Boltzmann constant
The diffusion coefficient of cation
The diffusion coefficient of anion
Absolute temperature
The number of charge carriers stoichiometrically related
to the salt composition.
ð9Þ
The ionic mobility of cations and anions of all the samples
has been calculated using the following equations:
μ ¼ μþ þ μ− ¼
0.925
0.50
1
0.29
1.42
270
The highest conducting sample in 70 PAN:30 NH4Br polymer
electrolyte system was used as an electrolyte for battery fabrication. Preparations of the anode desired proportions (3:1:1)
of zinc metal powder, ZnSO4·7H2O, and graphite powder
were taken and mixed together and finally ground well. Then,
the mixture was passed to form a thin pellet.
Preparation of the cathode the ratio of (8:2:1:0.5) PbO2,
V2O5, graphite, and polymer electrolyte was taken and mixed
together and finally grind well. The above mixture was made
into thin pellet. Graphite was added to introduce the electronic
conductivity, while the addition of the polymer electrolyte
helps in reducing the electrode polarization [64]. The polymer
electrolyte was sandwiched between the anode and cathode
pellets. The open circuit voltage (OCV) of the cell was
Ionic mobility and diffusion coefficient of cations and anions
Sample
n (cm−3)
t+
D+ (cm2 s−1)
D− (cm2 s−1)
μ+ (cm2 v−1 s−1)
μ− (cm2 v−1 s−1)
95 PAN:05 NH4Br
90 PAN:10 NH4Br
85 PAN:15 NH4Br
80 PAN:20 NH4Br
75 PAN:25 NH4Br
70 PAN:30 NH4Br
65 PAN:35 NH4Br
7.47×1020
1.49×1021
2.24×1021
2.99×1021
3.73×1021
4.48×1021
5.23×1021
0.92
0.94
0.95
0.95
0.97
0.99
0.95
2.61×10−08
3.69×10−08
4.28×10−08
5.88×10−08
6.07×10−08
8.99×10−08
7.87×10−08
2.22×10−09
2.31×10−09
3.22×10−09
4.35×10−09
6.59×10−09
8.75×10−09
4.67×10−09
1.00×10−06
1.42×10−06
1.49×10−06
1.69×10−06
1.95×10−06
3.45×10−06
2.41×10−06
6.03×10−08
6.54×10−08
7.35×10−08
7.85×10−08
8.03×10−08
9.43×10−08
7.91×10−08
J Solid State Electrochem
monitored for 96 h with configuration Zn+ZnSO4·7H2O/70
PAN:30 NH4Br/PbO2+V2O5.
Anode reaction
n Zn
þ ZnSO4 ⋅7H2 O ⇄ Znnþ1 ðSO4 Þ⋅ð7−2nÞH2 O⋅2n ðOHÞ
þ 2NHþ þ 2ne−
charges build up at the interface between the material and the
electrodes. From loss tangent spectra, low relaxation time
29.05×10−03 s has been observed for 70 PAN:30 NH4Br polymer electrolyte system. From the transference number measurement, it is clear that PAN:NH4Br polymer electrolyte is a proton
conductor where the value of μ+ and D+ is found to be higher
the value of μ− and D−. The primary proton-conducting battery
has been fabricated and their main parameters reported.
Cathode reaction
PbO2 þ 4Hþ þ 2e‐ ⇄ Pb2
þ
V2 O5 þ 6Hþ þ 2e‐ ⇄ 2VO2
þ 2H2 O
þ
þ 3H2 O
The OCV of a proton cell is described as the difference
between the equilibrium potentials at each electrode, with the
positive and negative electrode potentials (Fig. 13, battery
holder).
The stabilized voltage of 1.42 V observed for the cell
is shown in Fig. 14. The discharge characteristics of
stabilized voltage cell at room temperature for constant
load 1 M ohm is presented Fig. 15. This figure shows
the cell potential decreasing during discharge. The initial sharp decrease in voltage of this cell may be due to
polarization [65]. While discharging through 1 M ohm
load, the voltage value of cell remains constant at 1.3 V
for 270 h. The region in which the cell voltage remains
constant is called as plateau region. Beyond the plateau
region, voltage value of the cell drops again. The OCV
and discharge time for the plateau region and other cell
parameters for this cell are listed in Table 6.
Conclusion
The XRD spectrum confirms the amorphous nature of the
polymer electrolytes. The polymer–salt complex formation
has been confirmed from FTIR spectral studies. DSC studies
indicate that the glass transition temperature is low for the 70
PAN:30 NH4Br polymer electrolyte system. The ac conductivity value for all the prepared polymer electrolytes has been
calculated using impedance spectroscopic analysis at different
temperatures. The highest ionic conductivity has been found to
be 2.5×10−3 S cm−1 for 30 mol% of NH4Br doped with PAN
polymer electrolyte. The temperature dependence of ionic conductivity of these electrolytes exhibited Arrhenius behavior.
The low frequency dispersion of dielectric constant reveals
the space charge effects arising from the electrodes. The high
dielectric loss values at lower frequencies are due to the free
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