AC Impedance Spectroscopic of Nano Size Al2O3 Filler in PEO:AgI Polymer Electrolyte

Electrical and electrochemical properties of PEO based hot pressed nanocomposite polymer electrolyte (1-x)[70PEO:30AgI]:xAl2O3 where 0d≤x 10 wt% , have been studied. The conventional Solid Polymer Electrolyte composition (70PEO:30AgI) identified as highest conducting film at room temperature, has been used as 1st phase host matrix and nano-size particle of Al2O3 as 2nd phase dispersion. As a consequence of dispersal of Al2O3 in host, a conductivity enhancement of was achieved in film 93[70PEO:30AgI]:7Al2O3. These composition has been referred to as optimum composing composition. The ion transport behavior in polymer membrane have been discussed on the basis of ionic conductivity(σ), relaxation time (τ), and ionic transferred number(tion). The Impedance Spectroscopic study analysis at different temperature have been carried out to characterize the ion transport mechanism. The temperature dependence conductivity study has also been done to compute the activation energy(Ea) form logσ – 1/T plot.


INTRODUCTION
The ionic conducting Solid Polymer Electrolyte (SPEs) show tremendous technological power sources viz.flexible, compact, light-weight, leak-proof, thin film micro-batteries of desirable shape/size [1][2][3][4] .The conduction mechanism in polymer electrolyte was reported for the first time in 1973 5 , while the first practical SPE battery based on 'Poly(ethylene oxide) (PEO)-Li + -ion salt complex' was demonstrated in 1979 6 .Since then, a wide variety of SPEs, involving different kinds of mobile ions viz.Li + , H + , Na + , K + , Ag + etc. as principle charge carriers, has been reported.PEO-based polymer electrolyte is one of the most promising materials due to their good mechanical and electrochemical properties.SPEs are prepared usually by solution -cast method.It has been observed in general that the existence of high degree of amorphousity in polymer host(viz.PEO) supports high ionic mobility and hence, high ionic conduction in SPEs.However, the SPE films prepared conventionally be complexing the salts in polymeric host, are found to be less stable mechanically.In a recent investigation, it has been observed that mechanical strength of polymer electrolyte membranes as well as the degree of amorphousity in polymer host could be enhanced substantially by dispersing nano-size particles of an inert/insulating material such as Al 2 O 3 or SiO 2 or TiO 2 etc. [7][8][9][10][11][12] .Such system are referred to as nanocomposite polymer electrolytes(NCPEs).The size of the filler particles plays a significant role in improving these physical properties of SPEs.Very recently, in place of traditional solution casting method, a novel hotpress technique has been developed to prepare SPE membrances [10-14].The present paper report some electrictrical viz.ionic conductivity(σ), ionic transference number (t ion ), relaxation time (τ), activation energy (E a ) and electrochemical properties of hot-press synthesized new Ag + ion conducting nano-composite polymer electrolyte(NCPE) membrane (1-x)[70PEO: 30AgI]: xAl 2 O 3 where 0d³xd³10 wt% using different experimental technique.

Experimental Procedure
In present studies the impedance measurement technique has been employed to evaluate the true bulk conductivity and activation energy of polymer electrolyte film as a function of salt concentration and temperature.The highest conducting composition 70PEO:30AgI taken as based system to study the influence of Al 2 O 3--as a addition on it.The general formula for SPE system is (1-x)[70PEO:30AgI]:xAl 2 O 3 where 1≤x≤10 wt.(%), AR grade chemicals PEO (6x10 5 MW, Aldrich, USA,puerty 98%), AgI(purity 98%) and Al 2 O 3 (purity e≥99.8%,size<50nm) were used for the synthesis of hot-pressed NSPEs.Dry powder of constituent chemicals in appropriate wt(%) ratio were homogeneously mixed for 10 min.at room temperature then heated separately about 110 0 C for 20 min.at melting point of PEO and to remove moisture.It result into a soft lump/slurry which was then pressed (~ 2 ton/cm 2 ) between two SS-cold block giving rise to a uniform film.The polymer electrolyte film so obtain sandwiched between fine brass blocking electrode then it placed in a temperature regulated furnace.The impedance and corresponding phase angle θ were measure at different constant temperature for different composition using HIOKI 3532-50 LCR meter over frequency range 50Hz to 5MHz .The value of Bulk resistance R b determine by Cole-Cole plot from which conductivity is calculated.The activation energy was calculated by the slop of Arrhenius plot.The relaxation time, capacitance and dielectric constant of OCC was calculated by impedance spectroscopic graph at maximum frequency obtain on intersecting point of real and imaginary part of impedance in frequency vs impedance plot.The ionic transference number (t ion ) were determine by Waganer's polarization method.

RESULTS
At room temperature measurement values of ionic parameter σ, t ion , and activation energy E a of different composition are listed in Table 1.

Ionic transference number
The transference number (t ion ) were determine by dc-polarization (Waganer's polarization method) [15].The electrolyte sample was sandwiched between two brass blocking electrode and it was polarized by applying the d.c.potential 300 mV.The current response of the cell was measured by digital electrometer.A steady state current reached after ~1.5 hour.The ionic transference number calculated from the polarization current versus time plot (Fig. 5) using equation ..

.(3)
Where I i is the initial current and I f is the final residual current.
The transference number values for all the samples were found to be in the range of 0.95-0.99,which is close to unity indicate that the electrolyte are principally ionic conductors and that the charge transport is mainly Ag + ion.

Impedance Spectroscopic Study of Sample
The experimentally measured parallel values of the resistance and the capacitance were converted to their equivalent resistance and are displayed in the form of complex impedance (Z * = Z' -iZ") plots [16-19].The complex impedance can be expressed by using the Debye expression.
... (4)   which may be established by considering the specimens, as a parallel circuit of a pure resistor and an ideal capacitor.The 0 τ and 0 ω are the relaxation time and the characteristic angular frequency, respectively.The complex impedance can be expressed by introducing a temperaturedependent factor into the Debye expression, i.e., the Cole-Cole expression [16,18].... (5)  The is the mean relaxation time, and the factor τ o n take values over the range 0 <n<1, indicating polydispersive multi-Debye type relaxation.If n is unity, then the relaxation is explained by Debye-type response 17 .The real and the imaginary parts of the complex impedance can be rewritten from Eq. ( 6) in the following forms: ... (6)   where the Zs and Z 0 are the static impedance and the impedance at very high frequencies, respectively.
From the real and the imaginary parts of the impedance relaxation it is observed that at increasing frequency, the real part Z' decreases while the imaginary parts Z" shows a peakand and with increasing temperature, the impedance peak of the imaginary part is observed and shifts toward the high frequency side.
The complex impedance plot of the 93[70PEO:30AgI]:7Al 2 O 3 exhibits one impedance semicircular arc.Thus, one impedance arc representative of the bulk properties of the grains (dc resistivity).The centers of the impedance semicircular arcs lie below real axis.Thus, the impedance relaxation can be explained by using the Cole-Cole response 16,18 .
The intercepting point of imaginary and real part corresponds to 20,21 .... (7)   ω max is the frequency at the maximum of the semicircular arc of cole-cole plot.

CONCLUSIONA
new solid polymer electrolyte membrane (1-x)[70PEO:30AgI]:xAl 2 O 3 with different wt% of Al 2 O 3 has been investigated for the purpose of fabricating solid state thin film electrochemical devices viz.capacitor, batteries.A novel hot-press technique has been used for the film casting.The maximum conductivity of NCPE-OCC obtain at 7% Al 2 O 3 .The ion transport mechanism of NCPE-OCC has been studied in terms of basic ionic parameters σ, E a , t ion , relaxation time τ etc. the impedance spectroscopy study of the NCPE-OCC explain the properties of material with frequency.The very low activation energy of NCPE-OCC shows very good conducting property of electrolyte, the value of t ion close to unity shows that the electrolyte is pure ionic.Variation in relaxation time shows increase in velocity of ion in electrolyte with increase in temperature.