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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.

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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.

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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.