Materials Research Bulletin 46 (2011) 2009–2015

Structure, morphology and ionic conductivity of solid polymer electrolyte

Arup Dey a, S. Karan a, Ashis Dey b, S.K. De a,*a Department of Materials Science, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, Indiab Department of Chemistry, Sarsuna College, Sarsuna Upanagari, Kolkata 700061, India

A R T I C L E I N F O

Article history:

Received 21 April 2010

Received in revised form 30 June 2011

Accepted 7 July 2011

Available online 19 July 2011

Keywords:

A. Polymers

B. Chemical synthesis

C. Differential scanning calorimetry

D. Infrared spectroscopy

E. Ionic conductivity

Abstract: Polyethylene oxide (PEO) complexed with potassium iodide (KI) is synthesized to investigate

the ionic conductivity of alkaline based polymer electrolytes. The structural and morphological

characterizations of the nanocomposite polymer electrolytes are performed by X-ray diffractometry

(XRD), atomic force microscopy (AFM), differential scanning calorimetry (DSC) and Fourier transform

infrared spectroscopy (FTIR) measurements. The ionic conductivity increases with the increase of KI

concentration up to about 20 wt.%. The effect of nanosized ceria (CeO2 � 10 nm) fillers on ionic

conductivity in PEO–KI polymer electrolyte is also carried out, keeping PEO to KI wt.% ratio 80:20 and

85:15. The result reveals that the addition of ceria nanoparticles enhances the conductivity by two orders

of magnitude. The presence of ceria at the highest concentration induces the same molecular

environment within PEO chain as that of undoped PEO. Temperature dependence of ionic conductivity

follows Arrhenius mechanism.

� 2011 Elsevier Ltd. All rights reserved.

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jo u rn al h om ep age: ww w.els evier .c o m/lo c ate /mat res b u

1. Introduction

In recent years, more and more attentions have been paid to thepolymeric materials due to their potential applications in solid-state batteries, fuel cell, smart windows, sensors and electrochem-ical devices [1–4]. Polyethylene oxide (PEO)-based polymerelectrolyte is the most attractive host because of its easyfabrication of thin film, sub-ambient glass transition temperature,higher order of mechanical stability and comparatively highmobility of charge carriers. PEO also has the ability to dissolve highconcentrations of a wide variety of metal salts to form polymerelectrolytes. The ionic conductivity of pure PEO is in the order of10�7 S cm�1 at ambient temperature [5,6]. It has been establishedthat ion transport occurs through amorphous region assisted bythe segmental motion of the polymer chain. High concentration ofcrystalline phase in pure PEO impedes the ionic conductivity andmakes it unsuitable for practical applications. The materials basedon alkali metal salts complexed with PEO, known as solid polymerelectrolytes, play a critical role to improve the order of conductivity[7]. Increasing salt concentration, high levels ionic conductivitycan be achieved but the major disadvantage of these polymerelectrolytes is poor mechanical strength and potential stability.Most of the earlier studies were concentrated on Li and Na salts toinvestigate ionic conductivity. The potassium (K) ion has largerionic radius (1.33 A) than Li and Na. Higher ionic radius reduces thelattice energy of K salts. The degree of salt solvation of K salt is

* Corresponding author. Tel.: +91 3324733073; fax: +91 3324732805.

E-mail address: msskd@iacs.res.in (S.K. De).

0025-5408/$ – see front matter � 2011 Elsevier Ltd. All rights reserved.

doi:10.1016/j.materresbull.2011.07.008

higher due to lower lattice energy. Moreover the softness of Kmetal widens technological applications. Considering these facts,KI is chosen as salt in the present study.

Many efforts are devoted to overcome these drawbacks and todevelop the polymer electrolytes with high ionic conductivity.Recently, it is reported that the addition of various nanosizedceramic fillers such as SiO2, Al2O3, TiO2 in polymer matrix enhanceionic conductivity and improve mechanical properties at ambienttemperature [8–13]. Most of the investigations on nanofiller addedpolymer electrolytes have revealed that Lewis acid–base interac-tion between the nanoparticles and the polymer electrolytereduces the crystallization and enhances dissociation of thesupporting salt so that more free ions are available for conduction.Being a strong Lewis acid, ceria can interact with PEO-basedpolymer electrolytes. In the present paper, we have reported theeffect of CeO2 nanoparticle on structural modification, morpholo-gy, thermal properties and ionic conductivity of PEO–KI polymerelectrolyte.

2. Experimental

Ammonium Ceric nitrate (NH4)2[Ce(NO3)6] (Qualigens, Mum-bai, India) and citric acid monohydrate, as par analytical grade,were used to prepare ceria (CeO2) nanoparticles by autoignitionprocess and the details were given elsewhere [14].

Pure polyethylene oxide (molecular weight, MW = 106) filmand different concentrations of PEO and potassium iodide (KI)complexed films were prepared in the wt.% ratios (95:5), (90:10),(85:15), (80:20) and (75:25) by solution-casting technique usingmethanol as solvent. The solution was thoroughly mixed and

(a)

2θ (Degrees)

(b)

(c)

(d)

(e)

15 20 25 30 35 40 45 50 55 60

Inte

nsity

(a.u

)

Fig. 1. X-ray diffraction patterns of (a) pure PEO; (b) PEO–KI (wt.% ratio 80:20) and

(c) 8 wt.% CeO2; (d) 25 wt.% CeO2 of PEO–KI–CeO2 composite polymer electrolytes

and (e) pure CeO2 nanoparticles.

A. Dey et al. / Materials Research Bulletin 46 (2011) 2009–20152010

stirred using a magnetic stirrer at room temperature until thebubbles disappeared. The resulting homogeneous solution waspoured into polypropylene petri dish and allowed to evaporateslowly in air at room temperature. The films were then vacuumdried at 50 8 C for 48 h to remove all traces of the solvent. The thinfilms obtained were then preserved in a vacuum desiccator. Thesolubility of PEO depends on the dielectric constant of solvent.Generally acetonitrile is used as a solvent for higher dielectricconstant, e = 36. The value of e for methanol is 32.5, which is veryclose to that of acetonitrile [15]. Good quality films of PEO–KIcomplex were obtained using methanol as solvent in the presentsynthesis.

The effect of CeO2 nanoparticles in PEO–KI system were alsoinvestigated keeping PEO to KI wt.% ratio 80:20 and 85:15 in orderto study the influence of KI amount on ionic conductivity in thenanocomposite. Methanolic solution of PEO and KI with therequired wt.% ratio was first thoroughly mixed and stirred using amagnetic stirrer at room temperature. When the bubbles weredisappeared and the polymeric solution became viscous, CeO2 wasadded in estimated amount (3, 5, 8, 10, 15, 20 and 25 wt.%) andmixed with continuous stirring. The resulting homogeneoussolution was poured into polypropylene petri dish and vacuumdried at 50 8 C for 48 h to remove all traces of solvent. The thin filmswere preserved in a vacuum desiccator.

X-ray powder diffraction patterns of the pure CeO2, pure PEO,PEO–KI and the PEO–KI–CeO2 composite polymer in the form offilms were recorded by high resolution X’Pert PRO Panalytical X-ray diffractometer in the range 15–608 using Cu Ka radiation.Atomic force microscopic (AFM) studies were performed using anAutoprobe CP Base Unit, Model no. AP0100. Differential scanningcalorimeter (DSC) was carried out on PerkinElmer Diamond DSC ata heating rate of 20 8C/min from �60 8 C to 80 8 C heating scanunder a constant flow (100 ml/min.) of nitrogen gas to avoid anycontact of atmospheric moisture. Pure indium was used fortemperature and enthalpy calibration of the instrument. TheFourier Transform Infrared (FTIR) spectra were obtained by acomputer interfaced Shimadzu FTIR-8300 spectrometer in thefrequency range of 1500–800 cm�1. In this experiment thegelatinous polymer solution was cast on a KBr palette and driedin vacuum. The ionic conductivity of the composite electrolytefilms was measured using stainless steel electrodes and Agilent4192A Impedance Analyzer. The sample was sandwiched betweentwo polished stainless steel blocking electrodes. The bulkresistance was determined from the cross point at high frequencyof frequency dependent impedance with the real axis. The ionicconductivity was calculated from the geometrical factors and bulkresistance. Temperature was monitored by Eurotherm tempera-ture controller Model no. 2404 using a thermocouple sensor. Theion transference number in the polymer electrolyte was measuredby a configuration of stainless steel/polymer electrolyte/stainlesssteel under a fixed dc voltage of 0.8 V. The resulting current wasmonitored as a function of time. After polarizing the electrolyte,the transport number, tion, was calculated from the initial current,I0, and the saturation current, IS using the equation,

tion ¼I0 � IS

I0(1)

The current as function of time was measured by KeithleyElectrometer, Model no. 6517A.

3. Results and discussion

X-ray diffraction patterns of pure PEO, PEO–KI (wt.% ratio80:20), PEO–KI–CeO2 nanocomposites and pure CeO2 are shown inFig. 1, respectively. In Fig. 1(a) characteristic crystalline peaks ofPEO are observed at 2u = 19.13 and 23.308 which are assigned to

(1 2 0) and (1 1 2) planes, respectively. These peaks for PEO–KI asdisplayed in Fig. 1(b) become less intense compared to pure PEO.The intensity of the peak at smaller angle is higher than that oflarger angle. This indicates the decrease in degree of crystallinityand complexation of PEO with KI salt. Interaction between the K+

ions and ether oxygen of PEO chains disrupts the previous orderedarrangements of the PEO chains, resulting in the reduction ofcrystallinity. XRD patterns of composite polymer electrolytes areshown in Fig. 1(c and d), respectively. It is found that fouradditional peaks appear at 2u = 28.4, 32.9, 47.3 and 56.18 in theCeO2 added composite polymer electrolytes. These are attributedto (1 1 1), (2 0 0), (2 2 0) and (3 1 1) planes of cubic calcium fluoritestructure of CeO2 (Fig. 1(e)), respectively. It is also observed thatwith the increase in the content of filler the intensity of peakscorresponding to CeO2 increases. The crystallite size of thenanofillers is estimated by Scherrer formula [16]. The initial sizeof CeO2 is found to be approximately 10 nm. The size of CeO2 doesnot vary with increase of concentration.

Two-dimensional AFM topographic images of pure PEO, PEO–KI(wt.% ratio 80:20) and CeO2 added composite polymer electrolytesare shown in Fig. 2(a–d), respectively. The image of pure PEO(Fig. 2(a) presents a crystallized network of regular spherulitesdeveloping spirals and branches of well-distributed surfacecontours. This is the evidence of semicrystalline nature of purePEO. AFM micrographs indicating changes in the PEO surfacemorphology with the addition of KI salt as shown in Fig. 2(b).Interaction of KI salt with the PEO destroys the regular crystallizednetwork and reduces the degree of crystallinity of the polymercomplex remarkably. Introduction of the CeO2 nanoparticles intothe polymer matrix produces dramatic morphological changes tothe host polymer electrolyte. When 20 wt.% of CeO2 nanoparticlesare added, the composite polymer electrolyte exhibits almost a flatsurface (Fig. 2(c). In contrast, Fig. 2(d) shows the development ofgranular morphology when the value of CeO2 content in thecomposite polymer electrolyte reaches 25 wt.%. This structuralmodification promotes different crystallization behavior in thepolymer electrolyte system. AFM pictures suggest that theinsertion of CeO2 produces different morphologies of thecomposite polymer electrolytes. The favorable microstructure

Fig. 2. Two-dimensional AFM topographic images of (a) pure PEO; (b) PEO–KI (wt.% ratio 80:20) and (c) 20 wt.% CeO2; d) 25 wt.% CeO2 of PEO–KI–CeO2 composite polymer

electrolytes.

(a)

Temperature (ºC)

(b)

(c)

(d)

Hea

t Flo

w

(e)

-60 -40 -20 0 20 40 60 80

Fig. 3. DSC traces of (a) pure PEO; (b) PEO–KI (wt.% ratio 80:20) and (c) 5 wt.% CeO2;

(d) 20 wt.% CeO2; (e) 25 wt.% CeO2 of PEO–KI–CeO2 composite polymer electrolytes.

A. Dey et al. / Materials Research Bulletin 46 (2011) 2009–2015 2011

provides conducting path way for the ions at the surface of CeO2

which can improve the ionic conductivity.Fig. 3 shows the typical differential scanning calorimetry traces

of pure PEO, PEO–KI (wt.% ratio 80:20) and different PEO–KI–CeO2

systems. Fig. 3 reveals that the endothermic peak of PEO isbroadened and the peak height decreases with the inclusion of KIand CeO2 into the polymer matrix. In each curve, the endothermictransition has been evaluated as the melting of the crystallinephase of the polymer. Different thermal parameters, such as theglass transition temperature (Tg), the melting temperature (Tm),the enthalpy of melting (Hm) and the degree of crystallinity (x) aredetermined from DSC curves and are listed in Table 1.

It is observed from Table 1 that incorporation of nanofiller CeO2

into PEO–KI salt complex does not result in any systematic changein Tg. The values of Tg are within the range of �52.88 C to �41.88 Cfor all the ceria concentrations tested. The glass transitiontemperature is very important and can be used to determinethe miscibility and strength of the molecular interaction within thepolymer complex. Below Tg the polymer chains are considered tobe static and become dynamic above it. It is observed that theaddition of CeO2 to PEO–KI complex causes a random change in Tg.

The melting temperature of pure PEO is found to decrease from�718 C to �588 C due to the addition KI salt as displayed in Table 1.At lower concentration of ceria, Tm also decreases but it increaseswith further increase of the filler content. The endothermic peakaround the melting temperature exhibits the presence of crystal-line phase of the specimen. The enthalpy of melting is determinedfrom the area of the melting curve. From Table 1 it is clear that Hm

decreases from 155 to 28.47 J/g in the entire composition range.Percentage of crystallinity is determined by assuming pure PEO as

100% crystalline. The percentage of crystallinity decreasesremarkably from 100 to 27 upon addition of KI and it reaches tothe lowest value of 18 for 20 wt.% CeO2 added composite polymerelectrolyte. However, further addition of nanofiller enhances thecrystallinity as evident from Table 1 for 25 wt.% CeO2 addedcomposite polymer electrolyte.

Fig. 5. Deconvolution of FTIR spectra for C-O-C band in (a) Pure PEO (b) PEO–KI

(wt.% ratio 80:20) (c) 5 wt.% CeO2 (d) 25 wt.% CeO2 of PEO–KI–CeO2 composite

polymer electrolytes in the region of 1200-1050 cm�1.

Table 1Composition and glass transition temperature (Tg), melting temperature (Tm),

change of enthalpy of melting (Hm) and percentage of crystalline (x) of different

PEO–KI–CeO2 composite polymer electrolytes.

Sample Tg (8C) Tm (8C) Hm (J/g) x (%)

Pure PEO – 71.1 155 100

PEO–KI �50.2 58.2 41.34 27

PEO–KI–CeO2 3 wt.% �51.3 56.4 34.76 22

PEO–KI–CeO2 5 wt.% �52.3 55.0 32.65 21

PEO–KI–CeO2 8 wt.% �52.5 54.6 33.24 21

PEO–KI–CeO2 10 wt.% �52.8 54.3 33.62 22

PEO–KI–CeO2 15 wt.% �48.6 56.3 32.03 21

PEO–KI–CeO2 20 wt.% �49.4 56.7 28.47 18

PEO–KI–CeO2 25 wt.% �41.8 59.7 32.87 21

A. Dey et al. / Materials Research Bulletin 46 (2011) 2009–20152012

Further characterization has been performed by Fouriertransform infrared spectroscopy because it is an important toolto investigate the interaction between polymer–salt complex andalso the effect of introducing nanofiller in it. Lewis acid–baseinteraction among the nanofiller, polyether matrix and alkali metalions influences the structure of the polymer electrolyte bychanging the position and intensity of the infrared vibrationalbands [17]. The FTIR spectra of PEO, PEO–KI (wt.% ratio 80:20) andPEO–KI–CeO2 composite polymer electrolytes are shown in Fig. 4.The absorption features of pure PEO shown in Fig. 4(a) in thespectral ranges of 800–1000 cm�1 and 1200–1500 cm�1 areobserved due to different vibrational modes of H–C–H bonds.CH2 bending and asymmetric rocking modes are at 842 cm�1 and963 cm�1. Moreover, the bands at 1242 cm�1 and 1281 cm�1 areattributed to CH2 asymmetric and symmetric twisting and those at1342 cm�1 and 1360 cm�1 are ascribed to CH2 wagging modes,respectively. Peaks at 1455 cm�1 and 1467 cm�1 are ascribed tosymmetric and asymmetric CH2 bending, respectively. As thesebands are very sensitive to macromolecular conformations, theyprovide evidence for the presence of crystalline PEO phase. All CH2

bands are broadened in PEO–KI complex as shown in Fig. 4(b). Twowell-defined peaks at 1342 cm�1 and 1360 cm�1 for pure PEOmerge into a wide single peak. For PEO–KI–CeO2 samples, thewidth of the peaks of CH2 bands changes with the increase of thefiller concentration.

The most prominent peak of C–O–C symmetric stretching modeappears at 1110 cm�1. C–C stretching and C–O–C asymmetricstretching vibrations yield peaks at 1031 cm�1 and 1147 cm�1,respectively. The presence of the crystalline phase is confirmed bythe appearances of triplet maxima of C–O–C stretching vibrationsat �1147, 1110 and 1061 cm�1 in the pure PEO. A quantitative

(a)1031

1455

Wavenumber (cm-1 )

(b)1110

(c)

(d)

(e)

Abs

orba

nce

(a.u

)

1500 14 00 1300 12 00 1100 10 00 900 800

8429631061114712421281

134213601467

Fig. 4. FTIR spectra of (a) pure PEO; (b) PEO–KI (wt.% ratio 80:20) and (c) 10 wt.%

CeO2; (d) 15 wt.% CeO2; (e) 25 wt.% CeO2 of PEO–KI–CeO2 composite polymer

electrolytes.

analysis has been performed by fitting the FTIR spectra toGaussian–Lorentzian product function with straight base line[18]. The maximum intensity has been normalised to unity. Fig. 5is a typical example of a fit of a Gaussian–Lorentzian productfunction to the experimental FTIR data in 1000–1200 cm�1. Themaximum area for pure PEO is found for peak at 1112 cm�1. Fullwidth at half maximum (FWHM) of the maximum is 26 cm�1. Therelative area with respect to the maximum area for other two peaksat 1094 and 1147 cm�1 are 50% and 28%, respectively. The value ofFWHM of the major absorption band of PEO–KI is 51 cm�1. Thisbroadening clearly indicates K+ ion coordination with ether oxygenof PEO.

The calculated area ratio as a function of CeO2 concentration isdepicted in Fig. 6. The position (1110–1112 cm�1) of themaximum does not differ with CeO2 content. The area ratios ofpeaks at 1078–1093 cm�1 and 1147 cm�1 increase up to 5 wt.%CeO2 concentration. The grain boundary effect of nanosized CeO2

changes the spectra in low concentration region of CeO2[13]. Anincrease of CeO2 amount from 5 to 15 wt.% causes a decrease ofarea ratios of 1078–1093 cm�1 and 1147 cm�1 bands. Lewis acidcentres on the surface of CeO2 nanoparticles interact with the baseoxygen of polyether chain. This acid–base interaction increases thegauche conform of PEO chain. With further increase of CeO2

content, the area ratios of 1078–1093 cm�1 and 1147 cm�1 bandsincrease. Higher valence state and ionic radius of rare earth Ce-ioncompared to alkali K-ion create a different chemical environment.Both Ce4+ and K+ cations are competing against each other tocoordinate with ether oxygen. The increase of area in theconcentration region 15–20 wt.% clearly suggests that Ce4+

interacts with ether oxygen of solvating K+. The most interestingresult is that the IR band of C–O–C vibration for 25 wt.% of CeO2

doped polymer electrolyte is almost identical to that of pure PEO.Here CeO2 nanoparticles act as epitaxial centers for the growth ofcrystalline phase of PEO. This result is consistent with the DSC

25201510500.00

0.25

0.50

0.75

1.00

1.25

1.50 1147 cm -1

1110-1112 cm -1

1078-1093 cm -1

Are

a R

atio

Wt. % of CeO2

Fig. 6. Variation of area ratio with CeO2 of the three peaks in the spectral range

1150–1080 cm�1.

0 5 10 15 20 25-6.5

-6.0

-5.5

-5.0

-4.5

log(σσ) (

S-cm

-1)

KI conc entration (wt.%)

Fig. 7. Variation of conductivity with KI salt concentrations in polymer electrolyte

complex at room temperature.

2520151050-5.0

-4.5

-4.0

-3.5

-3.0

-2.5 PEO:KI = 85:15 PEO:KI = 80:20

log(

σσ σσ) (S

-cm

-1)

CeO2 concentration ( wt.%)

Fig. 8. Variation of conductivity with CeO2 concentrations in the composite polymer

electrolyte complex at room temperature.

A. Dey et al. / Materials Research Bulletin 46 (2011) 2009–2015 2013

study of the composite electrolyte at the highest concentration ofCeO2. The melting temperature of crystalline PEO formed in thepresence of highest content (25 wt.%) of CeO2 is 59.7 8 C which ismuch lower than that of pure PEO as shown in Table 1. Thissuggests that the crystalline phase of PEO in nanocomposite isquite different from crystalline nature of pure PEO. Similarbehavior in FTIR spectrum with higher concentrations of nanosizedZnO and Al2O3 have also been found [13,19,20]. Overall, theinteractions among the three moieties of PEO, KI and CeO2 areassociated with the changes in intensity, shape and position ofthese stretching bands [21].

The conductivity of the polymer–salt complex with various KIcontents is determined from the complex impedance measure-ments. The bulk resistance (Rb) is determined from the intercep-tion of the high frequency semicircle with the real axis in thecomplex impedance (Z1 and Z2) plot. The conductivity (s) of thepolymer complex in the form of films is calculated from thefollowing relation:

s ¼ t

RbA(2)

where ‘t’ and ‘A’ are the thickness and area of the film, respectively.Fig. 7 displays the variation of log(s) as a function of KI saltconcentration at ambient temperature. The result shows that theconductivity of pure PEO is in the order of �10�7 S cm�1 andincrease gradually with increases of KI concentration. Significantincrease is found up to �15 wt.% of KI. The enhancement inconductivity is due to the increase of conducting ion concentrationin the system. The conductivity reaches the value of1.53 � 10�5 S cm�1 at room temperature with 20 wt.% of KI salt.When the salt goes to 25 wt.% the conductivity almost saturates andthe rate of increase in conductivity becomes slower upon furtheraddition of KI salt to the polymer. From the structural analysis it ispredicted that K+ ion is coordinated to five ether oxygens [22]. Iontransport occurs by hopping between preferred oxygen sites ofpolymer. The increase of salt concentration introduces more mobileions. As a result of it, conductivity increases with increase in saltconcentration. The saturation of conductivity at higher saltconcentration may be due to ion–ion interaction.

Ionic conductivity measurements are also carried out fordifferent wt.% ratios of CeO2 nanoparticle, keeping the wt.% ratioof PEO to KI as 85:15 and 80:20. The dependence of conductivity inlog scale with ceria concentration at room temperature is depictedin Fig. 8. The plot shows two conductivity maxima, one is at around5 wt.% and the other is at around 20 wt.% of CeO2 added compositepolymer electrolytes, unlike the conventional composite polymer

electrolytes, which usually show one maxima. A lot of recent workshave been reported the existence of two maxima in compositepolymer electrolytes [23–27]. The variation of room temperatureionic conductivity (s) as a function of CeO2 concentration are alsosummarized in Table 2.

The incorporation of nanosized filler creates large interface onthe host polymer salt. Nanocomposite materials exhibit high grainboundary effect. Recently it has been predicted that interface layeris more conducting than bulk region [28,29]. For CeO2 content lessthan 5 wt.%, PEO backbones appear as trans arrangements. The PEOsolvation toward K+ ions through such arrangements is weakcoordination, which favors ionic conduction [13,30]. The formationof percolating conduction path around nanosized CeO2 and thetrans conformation of PEO molecule enhance the ionic conductivi-ty of the composite polymer electrolyte for low content of CeO2.

Molecular motion of PEO chain strongly affects the movementof ions. Mobile ions diffuse through PEO cooperatively with thesegmental motion of PEO long chain molecule. Lewis acid–baseinteraction between the polar surface groups of CeO2 and theelectrolyte ions at high CeO2 content, resulting in the gauchearrangement of PEO backbone. The gauche coordination is astronger interaction of PEO with Lewis acid. Gauche conformationalso restrains the PEO segmental motion, and is not very favorablefor ion transport in solid polymer electrolytes at room tempera-ture. Strong Lewis acid–base interaction and reduction ofsegmental motion may lead to decrease of conductivity as theCeO2 concentration increases from 5 to 10 wt.%.

Table 2Composition and room temperature ionic conductivity (s) and activation energy

(Ea) of different PEO–KI–CeO2 composite polymer electrolytes.

Sample s (S cm�1) Ea (eV)

PEO–KI 1.53 � 10�5 0.38

PEO–KI–CeO2 3 wt.% 2.71 � 10�4 0.21

PEO–KI–CeO2 5 wt.% 7.19 � 10�4 0.16

PEO–KI–CeO2 8 wt.% 4.55 � 10�4 0.17

PEO–KI–CeO2 10 wt.% 3.17 � 10�4 0.13

PEO–KI–CeO2 15 wt.% 1.16 � 10�3 0.12

PEO–KI–CeO2 20 wt.% 2.15 � 10�3 0.07

PEO–KI–CeO2 25 wt.% 6.89 � 10�4 0.11

2.9 3.0 3.1 3.2 3.3 3.4

-5.5

-5.0

-4.5

-4.0

-3.5

-3.0

-2.5

-2.0

(a) (e)(b) (f)(c) (g)(d) (h)

log(

σσ) (S

-cm

-1)

1000/T ( K-1 )

Fig. 9. Temperature dependence conductivity plots of (a) PEO–KI (wt.% ratio 80:20)

and (b) 3 wt.% CeO2; (c) 5 wt.% CeO2; (d) 8 wt.% CeO2; (e) 10 wt.% CeO2; (f) 15 wt.%

CeO2; (g) 20 wt.% CeO2; (h) 25 wt.% CeO2 of PEO–KI–CeO2 composite polymer

electrolytes.

0 100 200 300 400 500

0.0

0.1

0.2

0.3

Cur

rent

(mA

)

Time (min.)

Fig. 10. Polarization current versus time plot for 20 wt.% CeO2 of PEO–KI–CeO2

composite polymer electrolytes (PEO–KI wt.% ratio 80:20).

0 5 10 15 20 25 300.90

0.92

0.94

0.96

0.98

1.00

Ion

tran

sfer

ence

num

ber (

t ion)

CeO2 concentration (wt.%)

Fig. 11. Variation of ion transference number (tion) with CeO2 concentrations in the

composite polymer electrolyte complex (PEO–KI wt.% ratio 80:20) at room

temperature.

A. Dey et al. / Materials Research Bulletin 46 (2011) 2009–20152014

Above 10 wt.%, the high conducting regions are interconnectedto enhance the conductivity up to 20 wt.% CeO2 concentration.From FTIR analysis it is found that CeO2 interacts with etheroxygen of solvating K+ ion. This causes weak binding of K+ ion andimproves the conductivity. It is observed that the ionic conductiv-ity of 20 wt.% CeO2 added composite polymer electrolytes isimproved by an order of two in magnitude compared to that of thePEO–KI system at room temperature.

When the concentration of the CeO2 is high enough such as25 wt.%, the strong interaction with PEO induces more crystallinityin nanocomposites as evident from FTIR spectrum. The heteroge-neous nucleation by the epitaxial effect promotes crystalline phaseof PEO. Moreover, the segmental motion of PEO chain is alsoconfined by nanosized CeO2. Crystallinity and confinement ofsegmental motion of PEO result in a decrease in conductivity of thecomposite polymer electrolyte. The evolution of ionic conductivityas a function of CeO2 amount with 15 wt.% of KI (PEO:KI = 85:15)has also been studied and shown in Fig. 8. Similar type of variationof conductivity with ceria content is found for 15 wt.% and 20 wt.%of KI in the nanocomposite. An overall increase in conductivity isexpected to occur due to the high interface area between thematrix and the dispersed CeO2 particles [31–36]. The mostinteresting observation is that the variation of integrated intensityof two bands in the 1147 cm�1 and 1078 cm�1 of FTIR spectrum asshown in Fig. 6 reveal two maxima similar to conductivity as afunction of CeO2 concentration as depicted in Fig. 8. Thisestablishes that molecular interaction between PEO and inorganicfiller CeO2 significantly controls the ionic conductivity.

The temperature dependence of ionic conductivity of electro-lyte nanocomposites for different concentrations of CeO2 are

depicted in Fig. 9. The conductivity curves exhibit positivetemperature coefficient within the measured temperature range.The conductivity plots, log(s) vs. 1000/T, show linear behaviorwithin the studied temperature range. This behavior is consistentwith Arrhenius type charge conduction in polymer electrolytenanocomposites. The conductivity relationship is given as:

s ¼ s0exp�Ea

kT

� �(3)

where s0 is the pre-exponential factor, Ea is the activation energyand k is the Boltzmann’s constant. The activation energy ofindividual sample has been computed from the slope of eachstraight line and the data are listed in Table 2. The activationenergy of composite polymer electrolyte has been reducedconsiderably than that of polymer–salt complex.

The ion transference number (tion) of the polymer electrolytehas been calculated at room temperature by Wagner’s polarizationtechnique [37]. The sample was placed between a pair of stainlesssteel blocking electrodes. A typical plot of polarization current as afunction of time is shown in Fig. 10. The transference number fordifferent samples are calculated using equation (1). The value ofthe ion transference number for the polymer salt complex is �0.92whereas for 20 wt.% CeO2 added composite polymer electrolyte, it

A. Dey et al. / Materials Research Bulletin 46 (2011) 2009–2015 2015

reaches to �0.98. It is observed that the charge transport in thepolymer electrolyte is predominantly ionic. The variation of tion

with CeO2 concentration is shown in Fig. 11. Comparison betweenFigs. 8 and 11 clearly indicates that the ionic transference numbervaries in a similar manner as that of conductivity as a function ofCeO2 concentration. The observation of two maxima in the ionictransference number as displayed in Fig. 11 also confirms twodifferent type of interaction between PEO and nanosized ceria.

4. Conclusions

Polyethylene oxide polymer electrolyte nanocomposites areprepared by dispersing ceria nanoparticles in PEO–KI matrix. X-raydiffraction pattern of nanosized CeO2 indicates the presence ofcalcium fluorite structure. The ionic conductivity increases withthe amount of KI salt. The conductivity is further improved by theaddition of CeO2 nanoparticles. The maximum conductivity,2.15 � 10�3 S cm�1 has been obtained for 20 wt.% of CeO2 dopedcomposite polymer electrolyte. The incorporation of nanosizedCeO2 into PEO–KI salt matrix generates a heterogeneous complexsystem. Grain boundary effect of nanofiller, strong Lewis acid–baseinteraction between PEO and nanosized ceria, change of confor-mation (trans to gauche) of PEO molecule and epitaxial effect ofCeO2 nanoparticles play important roles to influence the ionicconductivity of composite polymer electrolyte.

Acknowledgement

This work is funded by the Council of Scientific and IndustrialResearch, Government of India, Scheme no: 03(1046)/05/EMR-II.

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