Effect of nanoadditives on ionic conductivity of solid polymer electrolyte

Indian Journal of Pure & Applied Physics Vol. 51, May 2013, pp. 281-288 Effect of nanoadditives on ionic conductivity of solid polymer electrolyte Arup Dey a, S Karan b & S K De c * a Garh Raipur High School, Raipur, Bankura 722 134 b Department of Physics, Narula Institute of Technology, 81 Nilgunj Road, Agarpara, Kolkata 700 109 c Department of Materials Science, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032 *E-mail: msskd @iacs.res.in Received 10 January 2013; revised 21 February 2013; accepted 6 March 2013 Solid composite polymer electrolytes consisting of high molecular weight polyethylene oxide (PEO) with potassium iodide (KI) and sodium perchlorate (NaClO 4 ) as electrolyte salts and cerium oxide (CeO 2 ) and zirconium oxide (ZrO 2 ) nanoparticles, respectively, as fillers have been prepared by standard solution cast technique. In the present work, PEO 15 -KI-CeO 2 electrolytes have been prepared over a range of CeO 2 content with particle size ~10 nm, whereas PEO 25 -NaClO 4 -ZrO 2 electrolytes have been synthesized with various particle sizes of nano ZrO 2 keeping the concentration constant to investigate the effect on the ionic conductivity. Keywords: Polymer electrolyte, Nanocomposite, Thermal properties, Ionic conductivity 1 Introduction Ionically conducting polymer composites are important materials both from fundamental studies as well as practical applications in high energy density solid-state batteries, supercapacitors, fuel cell, smart windows, sensors and electrochemical devices etc 1-4. The polymer-salt complexes, prepared by complexing polar polymer, particularly PEO, having high molecular weight and electron-rich heteroatom in its backbone with alkali metal salts namely, LiClO 4, NaClO 4, NH 4 ClO 4, KI etc., provide sufficiently high ionic conductivity 5-10. Ion transport occurs through amorphous region assisted by the segmental motion of the polymer chain. High concentration of crystalline phase in pure PEO impedes the ionic conductivity. Increasing salt concentration, high levels ionic conductivity can be achieved but the major drawback of these polymer electrolytes is poor mechanical strength and potential stability. Most of the recent research efforts to improve the room temperature conductivity without the fall of mechanical and potential stability have been directed towards the addition of nanoscale ceramic fillers such as SiO 2, Al 2 O 3, TiO 2, and CeO 2 into PEO based polymer electrolytes 2,11-17. A well accepted formation mechanism of composite polymer electrolytes is the Lewis acid-base reaction between the ceramic filler and the polymer 18,19. According to this mechanism, the interaction of Lewis acid sites on the surface of nanopartcles with base centers of ether oxygen in PEO leads to the formation of complex. The dispersed ceramic filler influences the recrystallization kinetics of the PEO polymer chains and ultimately establishes structural modification by promoting local amorphous phase within the polymer host. In addition, the Lewis acid-base type surface groups of filler interact with cations and anions and provide additional sites creating favourable high conducting pathways in the vicinity of filler grains for the migration of ions 20,21. The particle size of the filler is also expected to have a wide influence on the ionic conductivity of the composite polymer electrolytes. The conductivity increases with decrease in particle size, i.e., increasing specific surface area of the ceramic fillers. This may be due to stronger Lewis acid-base type interactions which enhance the dissociation of the supporting salt in the composite polymer electrolyte. However, the mechanism of ionic conductivity enhancement and the role played by the nanosized ceramic fillers are still not well understood. In the present work, the concentration-effect of CeO 2 nanoparticles in PEO 15 -KI system and the sizeeffect of ZrO 2 nanoparticles in PEO 25 -NaClO 4 system to improve the structural, morphological, thermal properties and ionic conductivity of these polymer electrolytes, have been reported. 2 Experimental Details 2.1 Preparation of CeO 2 Cerium oxide (CeO 2 ) powder was prepared by a citrate-nitrate autoignition process in which aqueous solution of ceric ammonium nitrate (NH 4 ) 2 [Ce(NO 3 ) 6 ]

282 INDIAN J PURE & APPL PHYS, VOL 51, MAY 2013 was mixed with an aqueous solution of citric acid maintaining a constant citrate to nitrate ratio (C/N). The mixed solution was allowed to evaporate at a fixed temperature (~ 200 C) with continuous stirring. The mixed solution first became viscous and turned into gel during heating. Then the gel slowly foamed, swelled and finally burnt to produce yellow-coloured CeO 2 powder. The powder was calcined at 200 C for 2 h and the average particle size was about 10 nm. 2.2 Preparation of ZrO 2 The nanosize zirconia was prepared using zirconium oxychloride octahydrate [ZrOCl 2, 8H 2 O] and NaOH by solvothermal process. For this, a closed cylindrical Teflon lined stainless steel chamber with a 50 ml capacity was taken. All the reagents and solvents were of analytical grade and used without any further purification. In a typical preparation process, 0.01 M zirconium oxychloride octahydrate [ZrOCl 2, 8H 2 O] was added to 0.05 M aqueous NaOH solution at room temperature and stirred for 20 min. A white precipitate was appeared, washed and dried at room temperature. The precipitate was then taken in a Teflon chamber filled to its 80% volume with ethanol. After 10 min of stirring the closed chamber was placed inside a preheated box furnace at desired temperature for 12 h. To calibrate the size of the nanostructures the experiment was performed at different synthesis temperatures such as 100, 120, 140 and 160 C. The precipitate was collected, washed with water and ethanol several times and dried in air at ambient temperature. 2.3 Preparation of PEO-based polymer electrolyte The solvent casting technique was used to prepare PEO 15 -KI and nanosize CeO 2 doped composite polymer electrolyte films using methanol as the common solvent. Methanolic solution of PEO (Mw=10 6 ) and KI with the required wt.% ratio was first thoroughly mixed and stirred using a magnetic stirrer at room temperature. When the bubbles disappeared and the polymeric solution became viscous, CeO 2 was added in estimated amount and mixed with continuous stirring. The resulting homogeneous solution was poured into polypropylene Petri dish and vacuum dried at 50 C for 48 h to remove all traces of solvent. The thin films were preserved in vacuum desiccators. For the preparation of PEO 25 -NaClO 4 and ZrO 2 added composite polymer electrolyte films, the same process was applied using the same solvent. The molar ratio in PEO-NaClO 4 polymer salt complex was 25:1. PEO and NaClO 4 were dried under vacuum at 50 C and 120 C, respectively, for 48 h before use. Prepared ZrO 2 nanometric powder with four different particle sizes were also vacuum dried at 120 C for 24 h prior to use. X-ray diffraction patterns of the polymeric systems were recorded by X Pert PRO Panalytical X-ray diffractometer in the range 15-65 using CuK radiation. The thermal behaviour of the electrolyte films was studied utilizing Perkin Elmer Diamond DSC in a temperature range-50-100 C with a scan rate of 10 C/min under a constant flow (100 ml/min) of nitrogen gas to avoid any contact of atmospheric moisture. Pure indium was used for temperature and enthalpy calibration of the instrument. The Fourier transform infrared (FTIR) spectra were obtained by a computer interfaced Shimadzu FTIR-8300 spectrometer in the frequency range 2000-500 cm 1. Ionic conductivity was measured using the ac impedance techniques with Agilent 4192A frequency response impedance analyzer, where the electrolyte films were sandwiched between two polished stainless steel blocking electrodes. Temperature was monitored by Eurotherm temperature controller (Model No. 2404) using thermocouple sensor. 3 Results and Discussion 3.1 The concentration-effect of CeO 2 nanoparticles in PEO 15 - KI system X-ray diffraction patterns of pure PEO, PEO 15 -KI, PEO 15 -KI-CeO 2 nanocomposites and pure CeO 2 are shown in Fig. 1. In Fig. 1(a), characteristic crystalline peaks of PEO are observed at 2 = 19.13 and 23.30 Fig. 1 X-ray diffraction patterns of (a) pure PEO; (b) PEO 15 -KI and (c) 8 wt.% CeO 2 ; (d) 25 wt.% CeO 2 of PEO 15 -KI-CeO 2 composite polymer electrolytes and (e) pure CeO 2 nanoparticles

DEY et al.: EFFECT OF NANOADDITIVES ON IONIC CONDUCTIVITY 283 which are assigned to (120) and (112) planes, respectively. These peaks for PEO 15 -KI displayed in Fig. 1(b), become less intense compared to pure PEO. The intensity of the peak at smaller angle is higher than that of larger angle. This indicates the decrease in degree of crystallinity and complexation of PEO with KI salt. Interaction between the K + ions and ether oxygen of PEO chains disrupts the previous ordered arrangements of the PEO chains, resulting in the reduction of crystallinity. XRD patterns of composite polymer electrolytes are shown in Fig. 1(c and d), respectively. It is found that four additional peaks appear at 2 = 28.4, 32.9, 47.3 and 56.18 in the CeO 2 added composite polymer electrolytes. These are attributed to (111), (200), (220) and (311) planes of cubic calcium fluorite structure of CeO 2 [Fig. 1(e)], respectively. It is also observed that with the increase in the content of filler the intensity of peaks corresponding to CeO 2 increases. The crystallite size of the nanofillers is estimated by Scherrer formula 22. The initial size of CeO 2 is found to be approximately 10 nm. The size of CeO 2 does not vary with increase of concentration. Two-dimensional AFM topoghaphic images of pure PEO, PEO 15 -KI and CeO 2 added composite polymer electrolytes are shown in Fig. 2(a d), respectively. The image of pure PEO [Fig. 2 (a)] shows a crystallized network of regular spherulites developing spirals and branches of well-distributed surface contours. This is the evidence of semicrystalline nature of pure PEO. AFM micrographs are indicating changes in the PEO surface morphology with the addition of KI salt as shown in Fig. 2(b). Interaction of KI salt with the PEO destroys the regular crystallized network and reduces the degree crystallinity of the polymer complex remarkably. Introduction of the CeO 2 nanoparticles into the polymer matrix produces drastic morphological changes to the host polymer electrolyte. When 20 wt.% of CeO 2 nanoparticles are added, the composite polymer electrolyte exhibits almost a flat surface as shown in Fig. 2(c). In contrast, Fig. 2(d) shows the development of granular morphology when the value of CeO 2 content in the composite polymer electrolyte reaches 25 wt.%. This structural modification promotes different crystallization behaviour in the polymer electrolyte system. AFM pictures suggest that the insertion of CeO 2 produces different morphologies of the composite polymer electrolytes. The favourable microstructure provides conducting path way for the ions at the surface of CeO 2 which can improve the ionic conductivity. Figure 3 shows the typical differential scanning calorimetric traces of pure PEO, PEO 15 -KI and different PEO 15 -KI-CeO 2 systems. Fig. 3 shows that the endothermic peak of PEO is broadened and the peak height decreases with the inclusion of KI and CeO 2 into the polymer matrix. Different thermal parameters, such as the glass transition temperature (T g ), the melting temperature (T m ), the enthalpy of melting (H m ) and the degree of crystallinity ( ) are determined from DSC curves and are listed in Table 1. Fig. 2 Two-dimensional AFM topographic images of (a) pure PEO; (b) PEO 15 -KI and (c) 20 wt.% CeO 2 ; d) 25 wt.% CeO 2 of PEO 15 -KI-CeO 2 composite polymer electrolytes Fig. 3 DSC traces of (a) pure PEO; (b) PEO 15 -KI and (c) 5 wt.% CeO 2 ; (d) 20 wt.% CeO 2 ; (e) 25 wt.% CeO 2 of PEO 15 - KI-CeO 2 composite polymer electrolytes

284 INDIAN J PURE & APPL PHYS, VOL 51, MAY 2013 Incorporation of nanofiller CeO 2 into PEO 15 -KI salt complex does not result in any systematic change in T g. The values of T g are within the range-52.8-41.8 C for all the ceria concentrations tested. It is observed that the addition of CeO 2 to PEO 15 -KI complex causes a random change in T g. The melting temperature of pure PEO is found to decrease from ~71 C to ~58 C due to the addition of KI salt as presented in Table 1. At lower concentration of CeO 2, T m also decreases but it increases with further increase of the filler content. The enthalpy of melting is determined from the area of the melting curve. From Table 1, it is clear that H m decreases from 155 to 28.47 J/g in the entire composition range. Percentage of crystallinity is determined by the formula, = H/ H PEO, where H PEO = 155 J/g assuming pure PEO as 100% crystalline. The percentage of crystallinity decreases remarkably from 100 to 27 upon addition of KI and it reaches to the lowest value of 18 for 20 wt.% CeO 2 added composite polymer electrolyte. However, further addition of nanofiller enhances the crystallinity as evident from Table 1 for 25 wt.% CeO 2 added composite polymer electrolyte. The FTIR spectra of PEO, PEO 15 -KI and PEO 15 -KI-CeO 2 are shown in Fig. 4. The absorption features of pure PEO are shown in Fig. 4(a) in the spectral ranges 800-1000 cm 1 and 1200-1500 cm 1 which are observed due to different vibrational modes of H-C-H bonds. CH 2 bending and asymmetric rocking modes are at 842 cm 1 and 963 cm 1. Moreover, the bands at 1242 cm 1 and 1281 cm 1 are attributed to CH 2 asymmetric and symmetric twisting and those at 1342 cm 1 and 1360 cm 1 are ascribed to CH 2 wagging modes, respectively. Peaks at 1455 cm 1 and 1467 cm 1 are ascribed to symmetric and Table 1 Composition and glass transition temperature (T g ), melting temperature (T m ), change of enthalpy of melting (H m ) and percentage of crystalline ( ) of different PEO 15 -KI-CeO 2 composite polymer electrolytes asymmetric CH 2 bending, respectively. As these bands are very sensitive to macro-molecular conformations, they provide evidence for the presence of crystalline PEO phase. All CH 2 bands are broadened in PEO 15 -KI complex as shown in Fig. 4(b). Two well-defined peaks at 1342 cm 1 and 1360 cm 1 for pure PEO merge into a wide single peak. For PEO 15 -KI-CeO 2 samples, the width of the peaks of CH 2 bands changes with the increase of the filler concentration. The most interesting result is that the IR band of C-O-C vibration for 25 wt.% of CeO 2 doped polymer electrolyte is almost identical to that of pure PEO. When the concentration of the CeO 2 is high enough both the cations, Ce 4+ and K +, are competing against each other to coordinate with ether oxygen of PEO. The hard Lewis acid centers on the surface of CeO 2 exhibit strong interaction with ether oxygen of PEO, even with ether oxygen of solvating K + ion. This, in turn, reduces the ability of salvation of PEO to KI salt and produces heterogeneous nucleation of PEO on the surface of CeO 2 nanoparticles. Thus the CeO 2 nanoparticles act as epitaxial centers for the growth of crystalline phase of PEO. This result is consistent with the DSC study of the composite electrolyte at the highest concentration of CeO 2. The melting temperature of crystalline PEO formed in the presence of highest content (25 wt.%) of CeO 2 is 59.7 C which is much lower than that of pure PEO as shown in Table 1. This suggests that the crystalline phase of PEO in nanocomposite is quite different from crystalline nature of pure PEO. Similar behavior in FTIR spectrum with higher concentrations of nanosized ZnO and Al 2 O 3 have also been found 23-25. Overall, the interactions among the three moieties of PEO, KI and CeO 2 are associated with the changes in intensity, shape and position of these stretching bands. Sample T g ( C) T m ( C) H m (J/g) (%) Pure PEO - 71.1 155 100 PEO 15 -KI 50.2 58.2 41.34 27 PEO 15 -KI-CeO 2 3 wt.% 51.3 56.4 34.76 22 PEO 15 -KI-CeO 2 5 wt.% 52.3 55.0 32.65 21 PEO 15 -KI-CeO 2 8 wt.% 52.5 54.6 33.24 21 PEO 15 -KI-CeO 2 10 wt.% 52.8 54.3 33.62 22 PEO 15 -KI-CeO 2 15 wt.% 48.6 56.3 32.03 21 PEO 15 -KI-CeO 2 20 wt.% 49.4 56.7 28.47 18 PEO 15 -KI-CeO 2 25 wt.% 41.8 59.7 32.87 21 Fig. 4 FTIR spectra of (a) pure PEO; (b) PEO 15 -KI and (c) 10 wt.% CeO 2 ; (d) 15 wt.% CeO2; (e) 25 wt.% CeO 2 of PEO 15 -KI- CeO 2 composite polymer electrolytes

DEY et al.: EFFECT OF NANOADDITIVES ON IONIC CONDUCTIVITY 285 The temperature dependence of ionic conductivity of electrolyte nanocomposites for different concentrations of CeO 2 is shown in Fig. 5. The conductivity curves exhibit positive temperature coefficient within the measured temperature range. The conductivity plots, log( ) versus 1000/T, show linear behaviour within the studied temperature range. This behaviour is consistent with Arrhenius type charge conduction in polymer electrolyte nanocomposites. The conductivity relationship is given as: = 0 exp( E a /kt) (1) where 0 is the pre-exponential factor, E a is the activation energy and k is the Boltzmann s constant. The activation energy of individual sample has been computed from the slope of each straight line and the data are listed in Table 2. The activation energy of composite polymer electrolyte has been reduced considerably than that of polymer-salt complex. 3.1 Size-effect of ZrO 2 nanoparticles in PEO 25 -NaClO 4 system All synthesized ZrO 2 samples were characterized by XRD at room temperature to identify the crystal structure. Diffraction peaks of all the samples were indexed to the cubic phase of zirconium oxide having space group Fm3m according to the JCPDS card no. 27-0997. The crystallite size of the sample was determined accurately by fitting the whole XRD pattern employing Rietveld method. The calculated values of the crystallite sizes are 4.5 ± 0.02 nm, 5.6 ± 0.05 nm, 6.4 ± 0.04 nm, and 7.6 ± 0:04 nm synthesized at temperature 100 C, 120 C, 140 C, and 160 C, respectively. XRD studies were carried out for pure PEO, PEO 25 -NaClO 4, composite polymer electrolyte films containing 4.5 and 7.6 nm of ZrO 2. Two characteristic peaks at 2 = 19.1 and 23.5 are assigned to (120) and (112) planes of crystalline PEO as shown in Fig. 6(a). The characteristic peaks of pure PEO for PEO 25 -NaClO 4 complex show variation in intensity and broadening as shown in Fig. 6(b) suggesting that the ordering of the PEO polymer is disturbed due to coordination interactions between the Na + ions and ether oxygen ions. In Fig. 6(c d), the appearance of crystalline peaks of ZrO 2 at 2 = 30, 50 and 60 confirm the incorporation of ZrO 2 nanoparticles in PEO 25 -NaClO 4 and the formation of composite polymer electrolyte. It is also observed that the peak intensity of ZrO 2 increases with the increase in particle size. The variation of glass transition temperature (T g ), melting temperature (T m ), change of enthalpy of melting ( H) and percentage of crystalline ( z ) of pure PEO, PEO 25 -NaClO 4 and different composite polymer electrolytes have been presented in Table 3. The value of T m decreases drastically nearly to 63 C from 71 C upon the addition of NaClO 4 into the Table 2 Composition and room temperature ionic conductivity ( ) and activation energy (E a ) of different PEO 15 - KI-CeO 2 composite polymer electrolytes Sample (S-cm 1 ) E a (ev) PEO 15 -KI 1.53 10-5 0.38 PEO 15 -KI-CeO 2 3 wt.% 2.71 10-4 0.21 PEO 15 -KI-CeO 2 5 wt.% 7.19 10-4 0.16 PEO 15 -KI-CeO 2 8 wt.% 4.55 10-4 0.17 PEO 15 -KI-CeO 2 10 wt.% 3.17 10-4 0.13 PEO 15 -KI-CeO 2 15 wt.% 1.16 10-3 0.12 PEO 15 -KI-CeO 2 20 wt.% 2.15 10-3 0.07 PEO 15 -KI-CeO 2 25 wt.% 6.89 10-4 0.11 Fig. 5 Temperature dependence conductivity plots of (a) PEO 15 -KI and (b) 3 wt.% CeO 2 ; (c) 5 wt.% CeO 2 ; (d) 8 wt.% CeO 2 ; (e) 10 wt.% CeO 2 ; (f) 15 wt.% CeO 2 ; (g) 20 wt.% CeO 2 ; (h) 25 wt.% CeO 2 of PEO 15 -KI-CeO 2 composite polymer electrolytes Fig. 6 X-ray diffraction patterns of (a) pure PEO; (b) PEO 25 - NaClO 4 and composite polymer electrolyte for (c) 4.5 nm, and (d) 7.6 nm size of ZrO 2

286 INDIAN J PURE & APPL PHYS, VOL 51, MAY 2013 polymer matrix. However, T m remains invariable after the nanofiller doping and it is also independent of the particle size of the filler. The value of T g as presented in Table 3, is not varying so much which indicates that the cross-linking and hence, mobility is not changing appreciably with doping of ZrO 2. The crystallinity ( z ) of the polymer nanocomposites has been calculated from the area of the melting curve using the formula, z = H/ H PEO, where H PEO = 213.7 J/g is the melting enthalpy of a completely crystalline PEO sample 26. The value of z in the PEO 25 -NaClO 4 -ZrO 2 system decreases remarkably with the addition of ZrO 2. The observed trend in PEO crystallinity can be explained as the Lewis acid-base type interaction among polyether matrix, ZrO 2 filler, Na + cation and ClO 4 anion 2,10,27. The complexation and ion-ion interactions between the various constitutions of PEO 25 -NaClO 4 -ZrO 2 complex have been studied by FTIR spectrum. It is found that a new peak at 625 cm 1 is appeared when NaClO 4 has been added into PEO matrix, which can be assigned to spectroscopically free (ClO 4 ) anions. This characteristic peak is also found to exist in all the composite polymer electrolytes. For all the samples, peak maximum of the (ClO 4 ) band in FTIR spectra has been normalized to unity and fitted to Gaussian- Lorentzian product function with a straight baseline 28. Fig. 7 is a typical example of fitted Gaussian- Lorentzian peak to the experimental FTIR data in the (ClO 4 ) region for PEO 25 -NaClO 4 -ZrO 2 composite system with 6.4 nm particle size of ZrO 2. It is observed that the (ClO 4 ) band has been well separated into two maxima centered at 624 and 633 cm 1. It is suggested that the (ClO 4 ) band within the ranges 620 and 625 cm 1 can be attributed to spectroscopically free ClO 4 anions whereas the band centered between 630 and 635 cm 1 is related to the contact-ion pairs 29,30. The fraction of free ClO 4 Table 3 Composition, size and glass transition temperature (T g ), melting temperature (T m ), change of enthalpy of melting ( H) and percentage of crystalline ( z ) of pure PEO, PEO 25 - NaClO 4 and different composite polymer electrolyte anions and Na + -CIO 4 contact-ion pairs has been calculated by taking the ratio of the integral area of each peak to the total area of (ClO 4 ) vibration. It is found that the value for free anions is much larger than that for contact-ion pairs. The percentage of free anions for PEO 25 -NaClO 4 system is 68 whereas for composite polymer electrolyte with 5 wt. % ZrO 2 of particle size 4.5 nm, it is found to be 81. For other composite polymer electrolytes with larger particle sizes of ZrO 2, however, a slight decrease in the fraction of free anions has been observed. This result suggests that the strong Lewis acid-base interaction with comparatively larger surface area of ZrO 2 nanoparticles favors the dissolution of NaClO 4 salt in the composite polymer electrolyte system and promotes more free Na + cations. The variation in ionic conductivity of composite polymer electrolytes as a function of the size of ZrO 2 nanofiller at room temperature is shown in Fig. 8. The optimum value of the ionic conductivity is 6.96 10 5 S-cm 1 for 4.5 nm of ZrO 2. The ionic conductivity Fig. 7 Peak fitting of FTIR spectra for (ClO 4 ) in composite polymer electrolyte for 6.4 nm size of ZrO 2. Sample Size of ZrO 2 (nm) T g ( C) T m ( C) H (J/g) z (%) Pure PEO - - 71.10 155 72.53 PEO 25 -NaClO 4 - -37.9 63.51 106 49.61 PEO 25 -NaClO 4 -ZrO 2 4.5-36.6 62.83 86 40.24 PEO 25 -NaClO 4 -ZrO 2 5.6-36.9 62.14 89 41.65 PEO 25 -NaClO 4 -ZrO 2 6.4-37.1 62.32 91 43.05 PEO 25 -NaClO 4 -ZrO 2 7.6-37.3 63.43 96 44.92 Fig. 8 Conductivity variation plot for different sizes of ZrO 2 nanoparticles at room temperature

DEY et al.: EFFECT OF NANOADDITIVES ON IONIC CONDUCTIVITY 287 Fig. 9 Temperature dependent conductivity plots of (a) PEO 25 - NaClO 4 and composite polymer electrolyte for (b) 4.5 nm; (c) 5.6 nm; (d) 6.4 nm; and (e) 7.6 nm size of ZrO 2 for (b) 4.5 nm; (c) 5.6 nm; (d) 6.4 nm; and (e) 7.6 nm size of ZrO 2 gradually decreases as the particle size of ZrO 2 increases and the lowest value of 4.64 10 6 S-cm 1 is obtained for 7.6 nm particle size. In PEO-salt complex, ion conduction occurs through amorphous regions. The nanosized ZrO 2 particles reduce the crystallinity of PEO chains resulting in higher conductivity. From Table 3, it is seen that the degree of crystallinity is the lowest for the smallest particle size which gives rise to the highest conductivity. Figure 9 shows the temperature dependence of ionic conductivity of PEO 25 -NaClO 4 and PEO 25 - NaClO 4 -ZrO 2 nanocomposites. All the samples show a discontinuity around T m of PEO in the temperature range 55-65 C. However, the degree of this discontinuity in the case of PEO 25 -NaClO 4 -ZrO 2 is obviously lower than that in the case of PEO 25 -NaClO 4 suggesting that the addition of ZrO 2 can, in part, restrain the recrystallization of PEO in the nanocomposite electrolytes, in accordance with the result obtained from XRD study. The maximum value of the ionic conductivity of PEO 25 -NaClO 4 - ZrO 2 is obtained for particle size 4.5 nm of ZrO 2 at all temperatures. This is possibly due to the relatively smaller size of ZrO 2 particles as compared to polymer host molecule which can easily penetrate into the polymer matrix and establish an interaction between ZrO 2 and polymer chain molecules. ZrO 2 particles have Lewis acid centers on their surface. The Lewis acid-base interaction of ZrO 2 with ether oxygen creates additional high conducting pathways for migrating ions. In addition, the enhanced surface area of ZrO 2 ensures the much stronger Lewis acid-base interactions between it and PEO chains, and thus a Fig. 10 Arrhenius plots of the conductivity of (a) PEO 25 - NaClO 4 and composite polymer electrolyte for (b) 4.5 nm; (c) 5.6 nm; (d) 6.4 nm; and (e) 7.6 nm size of ZrO 2 fitted by VTF Eq. (2) Table 4 Composition, size and fitted data of Vogel temperature, (T 0 ) and apparent activation energy (E A ) of PEO 25 -NaClO 4 and different composite polymer electrolyte Sample Size of ZrO 2 (nm) T 0 (K) E A (ev) PEO 25 -NaClO 4-194 0.027 PEO 25 -NaClO 4 -ZrO 2 4.5 201 0.011 PEO 25 -NaClO 4 -ZrO 2 5.6 198 0.022 PEO 25 -NaClO 4 -ZrO 2 6.4 198 0.023 PEO 25 -NaClO 4 -ZrO 2 7.6 199 0.025 small amount of ZrO 2 can effectively improve the ionic conductivity of the composite polymer electrolyte. As the number of such surface groups is expected to be proportional to the specific surface area of grains, our results appear to be consistent with this explanation. The migration of ion depends mainly on the segmental movement of polymer chain in the amorphous region. This type of ionic conduction often obeys empirical Vogel-Tammann-Fulcher (VTF) relation 31, (T) =AT 1/2 exp[ E A /k(t T 0 )] (2) where (T) is the total ionic conductivity and A is the pre-exponential constant proportional to number of carriers and T is the absolute temperature. E A is considered as the apparent activation energy. T 0 is the Vogel temperature or ideal glass transition temperature at which free volume of the polymer tends to zero value. The data above softening temperature for PEO 25 -NaClO 4 -ZrO 2 was fitted with the VTF equation, Eq. (2), by means of the usual approach of fitting and the fitted plot is shown in Fig. 10. The best fitted parameters are listed in Table 4. The estimated values of E A increase with

288 INDIAN J PURE & APPL PHYS, VOL 51, MAY 2013 increase of particle size. Comparison between Table 3 and Table 4 shows that the best fitted Vogel temperature T 0 is almost 50 C lower than the glass transition temperature (T g ) for each composition. The results are consistent with the literature 32-34. The close value of T 0 with T g and the presence of good linearity indicate that the ionic motion is coupled with the segmental motion of PEO chain. 4 Conclusions The ionic conductivity is remarkably improved by the addition of CeO 2 nanoparticles in PEO 15 -KI system. The maximum room temperature conductivity, 2.15 10 3 S-cm 1 has been obtained for 20 wt.% of CeO 2 doped composite polymer electrolyte. The epitaxial effect of CeO 2 nanoparticles plays important roles to influence the ionic conductivity of composite polymer electrolyte. Different sizes of cubic ZrO 2 nanoparticles have also influence the ionic conductivity. The room temperature ionic conductivity increases with decrease in particle size and the observed highest value of the ionic conductivity is 6.96 10 5 S-cm 1 for 4.5 nm of ZrO 2. Temperature dependence of conductivity above softening temperature follows empirical VTF theory. References 1 Vaia T J P & Beall G W, Polymer-clay Nanocomposites (Wiley, New York) (2002). 2 Croce F, Appetecchi G B, Persi L & Scrosati B, Nature, 394 (1998) 456-458. 3 Sasikala U, Kumar P N, Rao V V R N & Sharma A K, Int J Engg Sci & Advd Tech, 2 (2012) 722. 4 Singh P K, Nagarale R K, Pandey S P, Rhee H W & Bhattacharya B, Adv Nat Sci: Nanosci Nanotechnol, 2 (2011) 023002. 5 Reiter J, Krejza O & Sedlarikova M, Solar Energy Materials & Solar Cells, 93 (2009) 249. 6 Rajendran S, Babu R S & Sivakumar P, J Membrane Sci, 315 (2008) 67. 7 Bhide A & Hariharan K, Eupn Poly J, 43 (2007) 4253. 8 Zhou S & Fang S, Eupn Poly J, 43 (2007) 3695. 9 Reddy M J, Kumar J S, Rao U V S & Chu, Solid State Ionics, 177 (2006) 253. 10 Kalaignan P, Kang M S & Kang Y S, Solid State Ionics, 177 (2006) 1091. 11 Croce F, Curini R, Martinaelli A, Persi L, Ronci F, Scrosati B & Caminiti R, J Phys Chem B, 103 (1998) 10632. 12 Sun Ji K, Moon H S, Kim J W & Park J W, J Power Sources, 117 (2003) 124. 13 Nan C W, Fan L, Lin Y & Cai Q, Phys Rev Lett, 91 (2003) 266104. 14 Yuan A & Zhao J, Electrochim Acta, 51 (2006) 2454. 15 Morita M, Noborio H, Yoshimoto N & Ishikawa M, Solid State Ionics, 177 (2006) 715. 16 Wang X L, Mei A, Li M, Lin Y H & Nan C W, J Appl Phys, 102 (2007) 054907. 17 Dey A, Karan S & De S K, Solid State Ionics, 178 (2008) 1963. 18 Croce F, Persi L, Scrosati B, Serraino-Fiory F, Plichta E & Hendrickson M A, Electrochim Acta, 46 (2001) 2457. 19 Chung S H, Wang Y, Persi L, Croce F, Greenbaum S G, Scrosati B & Plichta E, J Power Sources, 97 (2001) 644. 20 Marcinek M, Bac A, Lipka P, Zaleska A, Zukowska G, Borkowska R & Wieczorek W, J Phys Chem B, 104 (2000) 11088. 21 Wieczorek W, Florjanczyk Z & Stevens J R, Electrochim Acta, 40 (1995) 2251. 22 Guinier A, X-Ray Diffraction in Crystals, Imperfect Crystals and Amorphous Bodies (Dover Publications, New York) (1994). 23 Zhang H, Wang J, Zheng H, Zhau K & Zhau Y, J Phys Chem B, 109 (2005) 2610. 24 Strawwhecker K & Manias E, Chem Mater, 15 (2003) 844. 25 Wieczorek W, Zaleswska A, Raducha D, Florjanczyk Z & Stevens J R, J Phys Chem B, 102 (1998) 352. 26 J Xi and X Tang, Chem Phys Lett, 393 (2004) 271. 27 Ibrahim S, Md R Johan, Int J Electrochem Sci, 7 (2012) 2596. 28 Irish D E, Tang S Y, Talts H & Petrucci S, J Phys Chem, 83 (1979) 3268. 29 Salomon M, Xu M Z, Eyring E M & Petrucci S, J Phys Chem, 98 (1994) 8234. 30 Wieczorek W, Zalewska A, Raducha D, Florjanczyk Z, & Stevens J R, J Phys Chem B, 102 (1998) 8725. 31 Ratner M A, Polymer Electrolyte Reviews, edited by J R Mac-Callum & C A Vincent, Elsevier, London, 1, (1989). 32 Wright P V, Electrochim Acta, 43 (1998) 1137. 33 Gray F M, Polymer Electrolytes, RSC Materials Monographs, The Royal Society of Chemistry, Information Services, UK, Letchworth, (1997). 34 Fontanella J J, J Chem Phys, 111 (1999) 7103.