Fig. 2a shows the AC conductivity of the Starch/PVALiBr nano composite polymer electrolytes at room temperature with different contents of silica nanoparticles. AC conductivity spectra have proved fruitful in exploring not only the electrical aspects but some structural rather micro structural aspects of disordered solid like nanocomposite polymer electrolytes. From the Fig. 2a it is discernable that the conductivity of the sample S-3 with 3 wt% of silica is highest among all the compositions. The increase in ac conductivity with frequency may be the outcome of the ionic motion facilitated by the conducting pathways formed by nano fumed silica. The ac conductivity spectrum of this composition shows dispersion in low frequency region and high frequency plateau region. The low frequency dispersion is attributed to the presence of
the well known electrode polarization phenomenon [23]. The plateau in higher frequency region for the sample S-3 is a signature of formation of conductive pathways that facilitate ionic motion and hence high conductivity is achieved. The high frequency plateau gives the true value of the dc conductivity [24,25]. The results are in good agreement with other nano composite polymer electrolyte systems [26,27]. The appearance of the high frequency plateau in the conductivity spectra is indicative of hopping of charge carriers from one site to the other thus contributing to the dc conductivity. The inability of the consecutive hops to fit into a time window given by roughly half a cycle is another factor that contributes to the appearance of the high frequency plateau in the conductivity spectra of disordered solids [28]. For the sample with 4 wt% silica the reduction in the plateau in high frequency region hence decrease of conductivity which is reflected in the bulk conductivity measurements. Inset of Fig. 2a shows the variation of the real part of impedance Z0 with frequency forthe Starch/PVA polymer electrolytes with different contents of silica nano particles. The decrease in the real part of the impedance supports the increase in ac conductivity [29]. The overall study of the ac conductivity spectra reveals the considerable effect of the silica nanoparticles in the polymer electrolyte samples. Fig. 2b shows the frequency dependent conductivity of the highest conducting sample S-3 with 3 wt% nano fumed silica for the temperature range 30–90 _C. The drastic increase in the conductivity at elevated temperatures is the outcome of the synergized contributions of the increased segmental motion and multiplication in the number of charge carriers. The high frequency plateau observed in the higher temperatures reflects the increase in dc conductivity.