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CHAPTER 5
STUDIES ON [PMMA/PVdF-HFP + SiO2] + NH4SCN + EC/PC
NANOCOMPOSITE GEL POLYMER ELECTROLYTE
Gel polymer electrolytes (GPE) have received considerable attention in the last few years as a
potential substitute of liquid electrolytes for their applications in various electrochemical devices
like rechargeable batteries, dye-sensitized solar cells, supercapacitors etc. The GPE systems
show a very high ionic conductivity normally in the range of 10-4
- 10-2
S cm-1
at room
temperature. A large number of GPEs based on polymer hosts like poly (vinylidenefluoride)
(PVdF), poly (vinylidenefluoride-co-hexafluoropropylene) (PVdF-HFP), Poly
(methylmethacrylate) (PMMA), poly (ethylene oxide) (PEO), poly (acrylonitrile) (PAN) etc.
and blends of these polymers, have been reported in the literature. An extensive review of GPEs
is given in section 1.2.1.5 in chapter 1. In general, gels are defined as a semi-solid material
consisting of an interconnected solid skeleton enclosing a liquid phase. The porous network of
gels filled with liquid electrolytes gives liquid like ionic transport while solid skeleton provides
mechanical support and stability. However, GPEs suffer from a few drawbacks like poor
dimensional stability, low liquid retention etc. Therefore many groups [223, 225, 235, 246, 247,
266, 290, 304, 395, 398, 399-402] have reported GPEs dispersed with nano-sized oxide ceramic
fillers like SiO2, Al2O3, TiO2 etc., to address the problem of mechanical stability and liquid
retention. They have found that ceramic dispersion improves the mechanical strength and
electrical conductivity of GPE systems. The incorporation of inert oxide nano fillers into the gel
polymer electrolyte network helps in maintaining the porous network of the polymers, thereby
assists in the higher liquid electrolyte uptake and also prevents the liquid electrolyte leakage
[223, 225]. An extensive review of nano ceramic filler dispersed gel electrolytes is given in
section 1.2.1.6 in chapter 1.
The present chapter describes the characterization of a nanocomposite GPE [35{(25 PMMA + 75
PVdF HFP) + x SiO2} + 65 {1M NH4SCN in EC + PC}], where x= 0, 1, 2, 4, 6, 8, 10, and 12,
where PVdF-HFP/PMMA blend is taken as a host polymer matrix, NH4SCN + EC + PC is taken
as liquid electrolyte and fumed silica (SiO2) is taken as ceramic filler. The nanocomposite GPE
system is prepared by solution cast technique which is discussed in details in chapter 2. The
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average particle size of the SiO2 is taken as ~ 9 nm. The structural, morphological and
electrochemical characteristics of the nanocomposite GPE has been carried out using various
techniques described in chapter 2. The results are described in the following sections.
5.1 Structural and Morphological Characterization
5.1.1 FTIR Studies
Figure 5.1 shows FTIR spectra of the blend GPE with and without SiO2 in the wavenumber
range 4000–650 cm−1
. The spectrum of blend GPE without SiO2 (figure 1a) contains vibrational
bands associated with the constituent polymers and the electrolyte.
Figure 5.1: FTIR spectra of (a) PMMA/PVdF-HFP blend GPE and its nanocomposite blend GPEs with, (b) 2 wt%
SiO2, (c) 6 wt% SiO2, and (d) 10 wt% SiO2.
The bands at 3220 and 1395 cm−1
are due to N–H stretching of ammonium ion and the bands at
2054 and 774 cm−1
are associated with the characteristic band of –SCN and S–C stretching,
respectively. The bands at 971 and 880 cm−1
are attributed to CH2 rocking and ring breathing
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modes of EC, respectively, and the bands at 944 and 715 cm−1
are assigned to the ring breathing
and symmetric ring deformation of PC, respectively. EC and PC also have broad bands in the
range 1822–1753 cm−1
and 1100–1020 cm−1
attributed to the C=O and C–O stretching
vibrations, respectively. A weak band at 1728 cm−1
corresponds to the interaction of cation
(NH4+) of the salt with C=O group of PMMA [395] and the bands at 1485 and 838 cm
−1 are
assigned to the CH2 bending and wagging vibrations, respectively. The spectrum also shows a
weak band at 1227 cm−1
, and two broad bands at 1202 and 1130 cm−1
which correspond,
respectively, to the out of plane bending vibration of vinylidene group, and –C–F– and –CF2–
stretching vibrations of PVdF-HFP.
Figure 5.2: Expanded FTIR spectra of (a) PMMA/PVdF-HFP blend GPE and its nanocomposite blend GPEs with,
(b) 2 wt% SiO2, (c) 6 wt% SiO2, and (d) 10 wt% SiO2.
142
The FTIR spectra of nanocomposite blend GPE with 2, 6, and 10 wt% SiO2 is shown in figure
5.1b–d. As can be seen, there is no significant change in the spectra of blend GPE in terms of
appearance of new bands or shifting of bands on dispersion of SiO2. However, on close
inspection, few changes in the shape of some of the bands have been identified. These changes
are shown in figure 5.2. The broad band between 1100 and 1020 cm−1
, corresponding to the C–O
stretching of EC and/or PC, broadens with SiO2 concentration. Oxides are known to have a
strong affinity towards EC/PC-solvent molecules [402]. Therefore, the broadening of C–O
stretching may be attributed to the strong affinity of EC/PC towards SiO2. This interaction may
lead to possible formation of silicone (Si–O–C) bond in the nanocomposite blend GPE which has
a broad characteristic vibrational band at 1100–1080 cm−1
. One more band, centered at 774 cm−1
due to S–C stretching vibration of SCN− ion of the salt has also been observed to receive similar
change on SiO2 dispersion. These observations show the ion–filler–polymer interaction in the
blend GPE.
5.1.2 XRD Studies
Figure 5.3 shows the XRD patterns of the nanocomposite blend GPE with different
concentrations of silica nanofiller.
Figure 5.3: XRD patterns of the blend GPE with silica concentration (a) 0, (b) 2 wt%, (c) 4 wt%, (d) 6 wt%, (e)
8 wt%, and (f) 10 wt%.
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The blend GPE without nanofiller has a semicrystalline characteristic with predominant
amorphous regions. The crystalline peak at 2θ= 20.2° superimposed over the broad halo
(corresponding to amorphicity) extending from 2θ = 10° to 60° corresponds to the characteristic
peak of PVdF-HFP. The intensity of this crystalline peak decreases with the increasing silica
concentration in the GPE. This shows the increase in the amorphicity of the GPE with nano filler
SiO2. The incorporation of silica nanoparticles in the blend GPE hinders the polymer chain
reorganization which results into significantly disordered polymer structure of nanocomposite
blend GPE resulting into higher amorphicity.
5.1.3 SEM Studies
In GPEs, electrical and electrochemical properties are governed mainly by two important factors,
namely network porosity and amorphicity of the polymer matrix. It is well known that dispersion
of inert nano-sized fillers in gel polymer electrolytes modify the pore network of the gel
polymers [247, 223, 225, 395, 400]. To see the possible changes in the surface morphology of
the blend GPE on the incorporation of silica nanoparticles, the SEM micrographs of the
nanocomposite blend GPE with 0-10 wt% SiO2 have been obtained. The SEM images are shown
in Figure 5.4. The polymer electrolyte network without SiO2 content shows a porous structure
with micron size porosity, as shown in Figure 5.4a. An addition of a small amount of nano-sized
SiO2 (~2 wt %) leads to a substantial change in the polymeric texture (Figure 5.4b) as the
morphology of the nanocomposite becomes more compact. The filler nanoparticles along with
the solvent EC/PC occupy micro-pores of the blend and modify the pore structure drastically.
The textural changes continue to occur as the SiO2 content is increased (Figure 5.4c and d).
Small sized SiO2 particles are not visible in SEM pictures, as if they are covered in the polymer
network. However, some bigger particles have segregated out from the polymer electrolyte
network and appear in the form of white spots in the SEM images (figure 5.4d) with 10 wt%
SiO2. These bigger particles are the aggregates of SiO2 particles.
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Figure 5.4: SEM images of blend GPE (a) without SiO2 and with (b) 2 wt% SiO2, (c) 6 wt% SiO2, and (d) 10 wt%
SiO2.
5.2 Ionic Conductivity Measurement
5.2.1 Composition Dependence of Ionic Conductivity
Figure 5.5 shows the variation of room temperature ionic conductivity of the nanocomposite
GPE, 35[(25 PMMA + 75 PVdF-HFP) + x SiO2] + 65(1M NH4SCN in 1:1 v/v EC-PC), as a
c
b d
a
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function of concentration of the silica nano filler. The maximum conductivity of the
nanocomposite GPE has been found as 4.3 × 10-3
Scm-1
, which is higher by a factor of ~3 than
the conductivity of GPE (1.3 × 10-3
Scm-1
) without filler.
Figure 5.5: Variation of room temperature ionic conductivity of blend GPE as a function of
concentration of SiO2 nano filler.
The conductivity variation further features two maxima, one for 2 wt% and the other for 8 wt%
concentration of the SiO2. Such type of conductivity variation is typical of composite polymer
electrolyte systems as reported earlier by many workers [304, 246, 247, 304]. The first
conductivity maximum is associated with the creation of free ions as a result of addition of filler
into the blend. The decrease in the conductivity beyond the first maximum is related to the
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formation of ion pairs and bigger sized ion clusters due to re-association of free ions when the
filler is added further. The second maximum in the conductivity pattern has been attributed to the
composite effect which relates the enhancement in the conductivity with formation of space
charge layer between SiO2 and the electrolyte and subsequent decrease in the conductivity due to
the blocking effect of the filler particles [290].
5.2.2 Temperature Dependence of Ionic Conductivity
Figure 5.6a shows the temperature dependence of ionic conductivity of the blend GPE with and
without nano filler measured between 25 °C to 80 °C. The conductivity shows a non-Arrhenius
characteristic, typical of the highly amorphous and solvent rich GPEs, which can be explained on
the basis of well known Vogel-Tamman-Fulcher (VTF) relation between conductivity and the
temperature:
0
21
expTT
BAT
where A is the pre-exponential factor, B is the pseudo-activation energy, and T0 is the
temperature close to glass transition temperature of the material. The conductivity variation with
temperature is consistent with the amorphous nature of the blend GPEs which is governed by
viscoelastic property of the polymers. The VTF behavior of conductivity may be associated with
the free volume generated due to expansion of polymer matrix on heating, which helps in the
high mobility of ions through the matrix and results in higher conductivity.
The ln(T1/2
) vs. 103/(T-T0) plots for the nanocomposite GPE with different concentrations of
silica nano filler are shown in Figure 5.6b. The parameters, A, B, and T0, have been evaluated by
non-linear least square fitting for each curve. The calculated values of these parameters are given
in table 5.1. As can be seen, the activation energy is minimum (B = 0.058 eV) for 2 wt% silica
concentration (the highest conducting composition) among the filler added blend GPEs. This
result is consistent with the blocking effect observed for higher filler concentration in
composition dependence of conductivity.
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Figure 5.6: Temperature dependence of conductivity of blend GPE (a) experimental (solid lines represent fitted
curves using VTF parameters), and (b) VTF fitted curves.
148
Table 5.1: VTF parameters of blend GPE for different concentrations of silica nano filler obtained by non-linear
least square fitting.
Concentration of SiO2 nano filler
(wt%)
Parameters
A (Scm-1
K-1
) B (eV) T0 (K) R2
0 0.084 0.014 279 0.9899
2 0.196 0.058 228 0.9899
6 0.116 0.118 232 0.9943
10 0.109 0.123 239 0.9996
5.3 Electrochemical Stability Window
The electrochemical stability window (ESW) indicates the working voltage range of electrolyte
membranes which is an important parameter of an electrolyte for its use in electrochemical
devices. The measurement of working voltage range was carried out by cyclic voltammetry on
the SS/nanocomposite GPE/SS cell.
Figure 5.7: Cyclic voltammogram of blend GPE with 2 wt% SiO2.
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Figure 5.7 shows a typical cyclic voltammogram of the nanocomposite GPE with 2 wt%
of SiO2. The volatmmogram does not show any reduction or oxidation peak in the
potential range from -1.6 V to +1.6 V. This gives an electrochemical stability window of
~3.2 V for the nanocomposite GPE which is a suitable range for device application
particularly as an electrolyte in a proton battery.
5.4 Ionic Transport Number (tion) Measurement
The ionic transference number (tion) of different compositions of the prepared
nanocomposite GPE has been evaluated by dc polarization method. A typical plot of the
current versus time for the highest conducting composition has been shown in figure 5.8.
For all the compositions, the ionic transference number lies close to unity, which shows
that the ions are the dominant charge carriers in the synthesized polymer electrolyte
system.
Figure 5.8: Polarization current versus time plot for nanocomposite blend GPE with 2 wt% SiO2.
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5.5 Confirmation of Proton (H+) Transport
In order to confirm the protonic conduction in the GPE system, complex impedance
spectroscopy and cyclic voltammetric studies have been carried out on the symmetrical
cells SS| nanocomposite GPE |SS (Cell-1) and Zn+ZnSO4.7H2O | nanocomposite GPE |
Zn+ZnSO4.7H2O (Cell-2). In Cell-1, the gel membrane is in contact with the stainless
steel (SS, a blocking electrode), whereas the pellets of Zn+ZnSO4.7H2O act as the
reversible electrodes (the proton source) in Cell-2. The comparative impedance plots for
Cell-1 and Cell-2 recorded at room temperature are given in figure 5.9.
Figure 5.9: AC impedance plots for; (a) Cell-1: SS | nanocomposite GPE |SS, and (b) Cell-2: Zn +
ZnSO4.7H2O | nanocomposite GPE | Zn + ZnSO4.7H2O.
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The impedance response of Cell-1 (with SS electrodes) shows the steeply rising nature of
Z” with frequency in the lower frequency range, which confirms the ion blocking nature
of the SS electrodes (Figure 5.9a). For Cell-2 (with reversible protonic electrodes), a
depressed semicircle is observed (Figure 5.9b). The appearance of well defined
semicircular in the impedance plot of cell-2 indecates that the Zn+ZnSO4.7H2O electrode
achieves equilibrium with the protonic species in the nanocomposite GPE and confirms
the protonic conduction.
In order to further confirm the proton conducting nature of the prepared nanocomposite
GPE, the CV plots for the two cells at a scan rate of 10 mVs−1
have been obtained which
are shown in figure 5.10.
Figure 5.10: Cyclic voltammogram for; (a) Cell-1: SS | nanocomposite GPE | SS, and (b) Cell-2: Zn +
ZnSO4.7H2O | nanocomposite GPE | Zn + ZnSO4.7H2O.
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The cathodic and anodic current peaks are distinctly observed for the cell-2 (with
reversible electrodes) (Figure 5.10b), whereas no such features are observed for Cell-1
(with SS electrodes) (Figure 5.10a). This kind of behaviour has also been observed in
various sodium (Na+) and magnesium (Mg
2+) ion conducting polymer electrolytes with
reversible electrodes [247, 246, 304]. Thus, in our case, the observation suggest that the
protonic oxidation and reduction take place at the respective electrode-electrolyte
interfaces of the Cell-2 and confirms the proton conduction in the nanocomposite GPE.
The reversible reaction at electrodes in this case is also same as described in section 4.6
of chapter 4.
Conclusions
The effect of nano filler (fumed silica) dispersion on the conductivity of PMMA/PVdF-
HFP-NH4SCN-EC-PC gel polymer electrolyte has been studied. Conformational changes
in the polymer network due to the interaction of liquid electrolyte and interaction of filler
particles with the blend GPE has been evidenced from the FTIR studies. SEM and XRD
studies indicate the micro-porous and amorphous nature of the nano-composite blend gel
polymer electrolyte system, responsible for higher conductivity. The variation of
conductivity with concentration of the filler shows two maxima which is typical of a
nanocomposite polymer electrolyte. The highest ionic conductivity of the blend GPE has
been obtained as 4.3 × 10-3
S cm-1
with 2 wt% of SiO2. The temperature dependence of
the conductivity shows the highly amorphous liquid like nature of the gel electrolyte
system. The electrolyte membranes have been found to be electrochemically stable in the
potential range from -1.6 V to 1.6 V. The ion transport number (tion) measurement shows
the dominant contribution of ions in the total conductivity of the prepared nanocomposite
blend GPE. The proton conducting nature of the prepared nanocomposite has been
confirmed from impedance spectroscopy and cyclic voltammetry.
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