Ionic Conductivity and Diffusion Coefficient of Barium Chloride Based Polymer Electrolyte with PSSA/PVA Polymer Complex

Transcription

1 Chapter 5 Ionic Conductivity and Diffusion Coefficient of Barium Chloride Based Polymer Electrolyte with PSSA/PVA Polymer Complex 5.1 INTRODUCTION The development of polymer electrolyte is a combinational study of electrochemistry, polymer science, organic and inorganic chemistry field. The synthesis of polymer electrolyte with good structural and electrical properties makes it suitable towards the development of electrochemical device fabrication (Saikia et al., 2006; Ketabi and Lian, 2012). In present scenario, the synthesis of small and highly rechargeable power sources polymer electrolyte is the main objective for industrial applications. Poly (4-styrenesulfonic acid) (PSSA) is a polymer contains a molecule obtained from polystyrene with sulfonic acid (functional group) is selected as a host polymer for present synthesis of polymer electrolyte. PSSA polymer is a combination of strong and weak acid- SO3- and COOH respectively, which reveals the anionic nature of polymer (Seo et al., 2009; Lin et al., 2007). Polyvinyl alcohol (PVA) has been used as cross binder for present investigation due to its well known properties as already discussed in previous chapters. The addition of organic/inorganic compound is a well known method to improve the basic physical properties of pure polymers. In present investigation a polymer electrolyte has been synthesized by incorporation of barium chloride dihydrate (BaCl2.2H2O) an inorganic compound into PSSA/PVA polymer system. The main objective of using BaCl2.2H2O with polymer matrix is to increase the final electrical properties of polymer electrolyte. Barium chloride dihydrate contains two water molecules which makes it better water soluble salt. In present study the structural, thermal and ionic properties of the synthesized composites were characterized. Temperature dependent ionic conductivity for different loading wt% of BaCl2 was investigated to identify the optimum conducting composition of PSSA/PVA/BaCl2 polymer electrolyte material. 91

2 5.2 EXPERIMENTAL MATERIALS The host polymer Poly (4-styrenesulfonic acid) (PSSA) of MW 75,000, product number was procured from Sigma Aldrich, Germany. PVA white granules of MW 1, 25,000 LR grade used as a copolymer was obtained from SD fine chem. limited, Mumbai, India. The inorganic compound barium chlorides dihydrate (BaCl2.2H2O) of MW , Batch T/ obtained from Sisco Research Laboratory, Mumbai, India SYNTHESIS OF BaCl2 BASED PSSA/PVA POLYMER ELECTROLYTE FILMS The samples were prepared by dissolving appropriate wt% of PSSA/PVA polymers in distilled water. The PSSA solution was stirred (350 rpm) at room temperature (30 oc) for 4 h until the mixture appears like homogeneous liquid. However, the PVA solution was stirred (350 rpm) at room temperature (60oC) for 8 h until the transparent solution will be obtained. Henceforth the Two individual solutions of PSSA and PVA was mixed together and kept for stir (350 rpm) at room temperature (30oC) for 6 h for proper intercalation between polymers and solvent. Simultaneously, the BaCl2.2H2O (5, 10, 15 and 20 wt%) was dissolved in distilled water and sonicated (for 10 min.) to achieve proper dispersion. Later on, the sonicated solution of barium chloride mixed with PSSA/PVA polymer solution and stirred (350 rpm) at room temperature (30oC) for 4 h. The prepared homogeneous solution was poured in the pettish dish and kept for drying for 12 h (at 60oC in oven). The prepared films have been kept in desiccators until no water left in the film. The proposed scheme of polymer composite electrolyte is shown in Scheme 5.1. In BaCl2 doped polymer electrolyte, inter/intra molecular hydrogen bonding occurs mainly between Ba2+ ions with the OH groups. The polymer composite films of thickness 50 µm was obtained from above procedure and used for further characterization. 92

3 Scheme 5.1: Proposed scheme of PSSA/PVA/BaCl2 polymer electrolyte 5.3 CHARACTERIZATION The structural characterization of BaCl2 doped polymer electrolytes was performed by using Cu Kα radiation of wavelength = Å produced by Bruker AXS D8 focus advance X-ray diffraction meter (Rigaku, Japan, Tokyo) with σi- filtered in the βθ range from 10-80o with a scanning speed and step size of 1o/min and 0.01o respectively. The complex chemical composition and functional group study of PSSA/PVA/BaCl2 polymer electrolyte were recorded using FTIR spectroscopy (make Shimadzu-IR Affinity-1 spectrometer) in the wave number range of cm-1 operated in a transmittance mode. The UV- visible measurement of PSSA/PVA/BaCl2 polymer electrolyte has been investigated by using Shimazdu UV-2401PC, UV vis. spectrophotometer in the range of nm. The optical absorbance of prepared polymer electrolyte was recorded and direct/indirect energy band gap has been evaluated from UV absorption spectrum. 93

4 Differential Scanning Calorimetry (DSC) experiments were performed by using DSC-60, Shimadju, Japan, for thermal characterization of polymer composite. The sample for DSC analysis was cut into small pieces of 2.5 mg weight from the original films. All the experiments were carried out at 350 C with heating rate of 10 C min 1 in air atmosphere. The electrical properties of PSSA/PVA/BaCl2 composite polymer electrolytes were demonstrated by impedance spectroscopy and ionic conductivity parameters. The sample (size 10 mm diameter, silver pasted both sides) was placed between fixture assembly and kept inside dry temperature calibrator. We selected the PSM-1735 impedance analyzer under varying range of temperature (40 to 150oC) and broadband frequency (50 Hz to 1 MHz). For result accuracy, all the samples has been tested three times and the average of it were drawn and analyzed. 5.4 RESULT AND DISCUSSION XRD ANALYSIS In order to investigate the effect of inorganic salt (BaCl2.2H2O) for different loading wt% on the structure of PSSA/PVA polymer system, XRD analysis has been performed. Fig. 5.1 (i and ii) represents the XRD analysis of pure and doped polymer electrolyte. 94

5 Fig 5.1: XRD Analysis of (i) Pure PVA, PSSA and BaCl2 and (ii) PSSA/PVA/BaCl2 for different concentration (a) 45:50:5, (b) 40:50:10, (c) 35:50:15 and (d) 30:50:20 loading wt%. The presence of broad humps confirms the amorphous nature of PSSA polymer and peaks for PVA reveals its semi-crystalline nature. Whereas, the sharp peaks shown in Fig. 5.1 (i) reveals the highly crystalline nature of BaCl2. The XRD analysis of polymer electrolyte for different loading wt% of BaCl2 is shown in Fig 5.1 (ii) (a-d). The decrease in relative intensity of apparent peaks was observed are directly proportional to loading wt% of salt. The results of PSSA/PVA/BaCl2 can be correlated and interpreted by considering Hodge et al, 1996 which establishes a relation between height of the XRD peaks and degree of crystallinity. The major changes in XRD analysis has been observed at 2θ= 19 to 20o for PSSA/PVA/BaCl2 complex polymer electrolyte. The XRD diffraction pattern represent more crystalline peaks for 10 wt% loading of BaCl2 based polymer electrolyte. This is due to proper incorporation of BaCl 2 ions in polymer electrolyte system. The evaluated structural parameters such as interplaner spacing (d) and average crystalline size (D) for pure and doped polymer system is tabulated in Table 5.1. The average crystalline size of the polymer electrolyte samples were evaluated by using Scherer s formulae as discussed in previous chapter. 95

6 Table 5.1: Structural parameters of PEG/PVA/LiOH polymer electrolyte from XRD analysis. Sample Code Pure PSSA Sample Details (wt%) 2θ (Degree) d (Å) D (nm) PSSA:: Pure PVA PVA:: A PSSA/PVA/BaCl2:: 45/50/ B PSSA/PVA/BaCl2:: 40/50/ C PSSA/PVA/BaCl2:: 35/50/ D PSSA/PVA/BaCl2:: 30/50/ The change in structural parameters is shown in Table 5.1. It can be noticed that the interplaner spacing (d) decreases and crystallinity increase with loading wt% of BaCl 2. In general, most of polymers substantially has a volume fraction and belongs to the amorphous phase. However, in present investigation, there are several crystalline peaks has been appear after adding BaCl2 in PSSA/PVA polymer system. The change represents the proper growth of salt in PSSA/PVA polymer complex. Whereas, the decrease in crystallinity for higher loading wt% of BaCl2 represents the amorphous nature due to better alignment of polymer chain which will influence the conductivity results FTIR ANALYSIS The FTIR analysis has been characterized to obtain the chemical compositional change specifically functional group change in polymer electrolyte system as function of different loading wt% of BaCl2 salt. The FTIR spectrum of PSSA/PVA/BaCl2 complex polymer electrolyte is shown in Fig The FTIR bands for PVA as shown in Fig. 5.2 (i) represents the intermolecular OH bending at 3348 c.m-1, C-H stretching at 2928 c.m1, C=O stretching at 1733 c.m-1, -OH bending (absorbed water) at 1640 c.m-1. The peak at 1438 c.m-1 and 1378 c.m-1 corresponds to wagging of CH2 vibration. The band at 916 and 850 c.m-1 represents the skeletal vibration. In addition of BaCl2, the FTIR peaks of pure PVA has been shifted to 3115, 2910, 1249, 1141, 947 and 850 c.m-1 as shown in Fig

7 (i). This shift in wavenumber is due to intercalation of BaCl2 with host polymer. The FTIR band of pure PSSA as shown in Fig. 5.2 (i) represents the strong absorption band centered at 1200 c.m-1 is an asymmetric stretching vibration of the O=S=O unit. Fig 5.2: FTIR Spectra of (i) Pure PVA, PSSA and PVA/BaCl2:: 50:50 and (ii) PSSA/PVA/BaCl2 (a) 45:50:5, (b) 40:50:10, (c) 35:50:15 and (d) 30:50:20 loading wt%. 97

8 The peak at 1005 c.m-1 results from the vibration of phenyl ring substituted with a sulfonic group. The broadening of this peak and overlapping with the phenyl ring attached with sulfonic anion peak located at 1125 c.m-1 indicates the deprotonation of the sulfonic acid peak. The FTIR spectrum of 5, 10, 15 and 20 wt% loading of BaCl2 with PSSA/PVA polymer system is shown in Fig 5.2 (ii). The polymer electrolyte with 5 wt% loading of BaCl2 represents the FTIR peaks at 3197 c.m-1 assigned for OH vibration, 2954 c.m-1 for C-H stretching, 1406 c.m-1 for C-H deformation and at 1325 c.m-1 assigned for C-O stretching as shown in Fig 5.2 (ii) (a). The peaks has been shifted to 3061, 2902, 1641, 1408 c.m-1 for 10 wt% loading of BaCl2 in polymer electrolyte corresponding to C-H stretching, C=O stretching vibration, -OH vibration and C-H deformation respectively as shown in Fig 5.2 (ii) (b). The drastic change in shift of FTIR peaks has been observed for higher loading wt% of BaCl2 in polymer electrolyte. The FTIR peaks for 20 wt% loading of BaCl2 was obtained at 3099 c.m-1 assigned for OH vibration, 2954 c.m-1 for C-H stretching and 1624 c.m-1 for OH bending. This implies the aggregation of multiple ions when loading wt% of BaCl2 increases in polymer electrolyte. These changes attributes to the interaction of salt ions with host polymer system UV-VISIBLE SPECTRSOCOPY ANALYSIS The UV-vis. study has been carried out to determine the optical band gap of PSSA/PVA/BaCl2 polymer electrolyte. According to optical absorption spectra and optical band gap values, the materials are distinguished into semiconductor and insulators. In present study the optical absorbance of pure and BaCl 2 doped polymer electrolyte in the range of nm is shown in Fig The inset of Fig 5.3 represents the optical absorbance spectra of pure polymer and pure inorganic compound. The absorption band observed in the region of 200 nm can be explained in terms of interaction or delocalization of the π-π* orbital. The shift in absorption band is obtained in accordance to different loading wt% of BaCl2 polymer electrolytes. The absorption peaks are related to the transition of electron. Therefore the shifting in UV band towards the lower or higher wavelength depends upon the transition of electrons (Al-Gashani et al., 2012; Said et al., 2015). 98

9 Fig 5.3: Optical absorbance spectra of PSSA/PVA/BaCl2 for (a) 45:50:5, (b) 40:50:10, (c) 35:50:15 and (d) 30:50:20 loading wt%. Inset represents the UV-vis. spectra of pure PSSA and BaCl2. To understand the absorption peak shift in UV spectrum, the optical band gap energy (Eg) has been evaluated. Term optical band gap explains the transition of electron from valence band to conduction band. The optical band gap (Eg) in present study is evaluated by using Tauc plot method. ℎ = ℎ Where, α is the absorption coefficient, h is the photo energy, n depends on the type of transition may be equal to 1/2, 2, 3/2 and 3 corresponding to different transitions. Fig. 5.4 (i and ii) represents the variation of (αh )1/2 and (αh )2 intercept in the h axis. The energy gap is obtained by fitting the linear part of the curve and finding the intersection of the straight line with the h axis. The estimated values of band gap energy (Eg) for BaCl2 doped polymer electrolyte is tabulated in Table 5.2. The band gap value for pure PSSA and BaCl2 is 5 and 5.97 ev respectively. However, the estimated direct band gap energy (Eg) value for 5-20 wt% loading of BaCl2 in polymer electrolyte is varied from ev respectively as shown in Table 5.2. The band gap energy (Eg) decreases in accordance to loading wt% of BaCl2. Whereas, the estimated indirect band gap energy 99

10 (Eg) value for 5-20 wt% loading of BaCl2 in polymer electrolyte is varied from ev respectively as shown in Table 2. Fig 5.4: Plots for (i) Direct optical band gap and (ii) Indirect optical band gap for PSSA/PVA/BaCl2 for (a)45:50:05, (b) 40:50:10, (c) 35:50:15 and (d) 30:50:20 loading wt%. 100

11 Table 5.2: Direct and Indirect energy optical band gap values evaluated from UV-vis. spectrum Sample Code Eg (ev) (Direct Band Gap) Eg (ev) (Indirect Band Gap) a Sample Details (PSSA/PVA/BaCl2 wt%) 45/50/ b 40/50/ c 35/50/ d 30/50/ The decreased value of band gap energy (Eg) indicates that the less energy is required to produce electron-hole pair which does not separate electron-hole. Hence the produced electron-hole pair is electrically attracted to each other. Therefore the evaluated band gap values can be directly correlated with the ionic conductive of polymer electrolyte OPTICAL MICROSCOPY STUDIES The surface studies of polymer electrolyte films in presence of BaCl2 are further characterized by polarized optical microscopy. The objective of optical microscopy characterization is to optimize the crystalline to amorphous phase transition occurs for higher salt concentration. The microscopic examination based on PSSA, PVA and BaCl 2 in different ration gives information about the homogeneity and phase transformation in the samples. Fig. 5.5 (a-e) provides the microscopic images of PVA/BaCl2 and PSSA/PVA/BaCl2 polymer composites. The homogeneity is observed in presence of BaCl2 with pure PVA as shown in Fig. 5.5 (a). The plane dark surface represents the host polymer system, whereas a tiny dark bubble patterns is due to BaCl2. Fig. 5.5 (b) represents PSSA/PVA/BaCl2 polymer electrolyte exhibits a plane non-volume filling pattern revealing a significant buildup of amorphous phase. This amorphous phase is obtained in presence of low loading wt% of BaCl2 in polymer electrolyte. This result can also be correlated with XRD results. However, for 10 wt% loading of BaCl 2 large amount of BaCl2 is distributed among PVA/PSSA polymer system as shown in Fig. 5.5 (c). The presence of large amount of BaCl2 exhibits the filled up volume fraction of polymer system which also reveals the semi-crystalline nature of polymer electrolyte. The strong 101

12 intercalation between PSSA, PVA and BaCl2 is shown in Fig. 5.5 (d). The bright tiny bubble patterns occupy some particular position on polymer surface represents the homogeneity and phase transformation. Furthermore, the polymer surface dominates the presence of BaCl2 as shown in Fig. 5.5 (e). These strong cross linking of polymers restrict the movements of BaCl2 ion across the surface. Hence the present result obtained from polarized optical microscopy gives the significant information regarding homogeneity of polymer electrolyte. Fig 5.5: Optical micrographs of (a) PVA/BaCl2:: 50:50 and PSSA/PVA/BaCl2 for (b) 45:50:5, (c) 40:50:10, (d) 35:50:15 and (e) 30:50:20 loading wt%. 102

13 5.4.5 DIFFERENTIAL SCANNING CALORIMETRY ANALYSIS The thermal properties are very important for polymer electrolyte to determine its melting temperature, glass transition temperature and crystalline temperature in view of implementation. The DSC thermograms of pure PVA, PSSA polymers and its controlled composites are shown in Fig. 5.6 (i and ii). Fig 5.6: DSC thermograms of (i) Pure PVA, PSSA and PVA/BaCl2:: 50:50 and (ii) PSSA/PVA/BaCl2 for different concentration (a) 40:50:10, (b) 35:50:15 and (c) 30:50:20 loading wt%. 103

14 The sharp endothermic peaks are observed at 190oC corresponds to crystalline melting temperature (Tm) of pure PVA as shown in Fig 5.6 (i). Whereas, a broad peak has been observed for pure PSSA which defines its amorphous nature and the result can also be verified by XRD graph. In addition of BaCl2 with pure PVA, several sharp peaks have been observed as shown in Fig 5.6 (i). The observed sharp peaks define the crystallinity of the system but reveals the partial immiscibility. The DSC thermograms of PSSA/PVA polymer electrolyte for different loading wt% of BaCl2 are shown in Fig 5.6 (ii) (a-c). There are several peaks obtained in DSC thermograms and we assumed that it might be due to the presence of water. Hence the films have been dried further more and repeated the DSC experiments. But the same results are examined. Therefore, we concluded that the peaks are not due to the presence of water. The increase in melting temperature was observed with respect to change in loading wt% of BaCl2 which also resultant the increase in crystallinity and immiscibility nature of polymer electrolyte. The melting enthalpy ΔHm estimated experimentally demonstrates the heating point at which substance change its state from solid to liquid. The ΔHm of pure PVA and PSSA/PVA/BaCl2 polymer electrolyte is tabulated in Table 5.3. Table 5.3: Thermal properties of a) pure PVA, b) pure PSSA, c) PVA/BaCl2:: 50:50 and (d) 40:50:10, (e) 35:50:15, (f) 30:50:20 is for PSSA/PVA/BaCl2. Sample Code Tm (oc) ΔHm (J/g) χc % a B C D E F The relative percentage of crystallinity (χc %) was calculated on the basis of following equation with the assumption that pure PVA was 100% crystalline. = %

15 Where, Δ is the standard enthalpy of fusion of pure PVA and ΔHm is enthalpy of fusion of the composite polymer electrolyte obtained directly from the DSC data. The χc, ΔHm and the crystalline melting temperature (Tm) of all polymer electrolytes is tabulated in Table 5.3. The crystallinity of polymer electrolyte is increases with respect to loading wt% of BaCl2. The highest crystallinity was observed for 15 wt% loading of BaCl2 in polymer electrolyte. The results are also confirmed from XRD data. The polymeric chains are not much flexible in crystalline phase. The less segmental motion of polymer could be the reason for immiscibility of polymer electrolyte (Nasir et al., 2010; Vargas et al., 2000). The change in degree of crystallinity can be due to coordination bonds and the ions present in BaCl2 which interrupts the packaging of polymer molecules. The resultant crystalline phase is expected to favor ion transport, which enhanced the conductivity as well as diffusion coefficient of polymer electrolyte IMPEDANCE SPECTROSCOPY ANALYSIS Impedance spectroscopy is a common technique for understanding the charge transport phenomenon occurs in polymer electrolytes. The impedance method is widely used to investigate the electrical behavior of materials over a wide range of temperature and frequency. Impedance spectroscopy separates the real and imaginary component of electrical parameters. Fig 5.7 (a-c) represents the Nyquist (Zꞌ vs. Zꞌꞌ) plot for different loading wt% of BaCl2 in polymer electrolyte at different temperature ( oc). The semicircular curve has been obtained by fitting the data in Z-view. The impedance spectrum for 5 wt% loading of BaCl2 shown in Fig. 5.7 (a) is characterized by the appearance of single semicircular arc whose radii is gradually decreasing with increase in temperature. The intercept of each semicircular arc with x-axis gives the value of bulk resistance. The drastic change in impedance spectrum has been obtained in addition of higher loading (10-20 wt%) of BaCl2 in polymer electrolyte as shown in Fig. 5.7 (b-c). The decrease in semicircular pattern has been obtained with increasing loading wt% of BaCl2 in polymer electrolyte. This is due to the strong intercalation between salt and polymer ions at higher concentration. Hence it requires high temperature for the mobility of ions. Only single semicircular arc at all temperature demonstrates the grain effect other than the grain boundary and electrode effect (Martos et al., 2015). The impedance 105

16 response of PSSA/PVA/BaCl2 polymer electrolytes is due to the single conduction mechanism. This result brings to the conclusion that the current carriers are ions in the present polymer electrolytes. 106

17 Fig 5.7μ Complex impedance plane plots (Zꞌꞌ vs. Zꞌ) of PSSA/PVA/BaCl2 for (a) 45:50:5, (b) 40:50:10 and (c) 35:50:15 loading wt% IONIC CONDUCTIVITY STUDIES The movement of ions occurs in polymer electrolyte are called as ionic conduction mechanism. The ionic conductivity property helps to produce better polymer electrolyte. The ion conduction mechanism in polymer electrolyte depends on various parameters such as concentration of salt and their mobility of medium (Kuan et al., 2015). In order to optimize the effect of BaCl2 in the polymer electrolyte on the basis of ionic conductivity the loading wt% of BaCl2 increased from 5 20 wt%. The change in ionic conductivity as a function of loading wt% of BaCl2 at different temperature is shown in Fig The highest room temperature conductivity 9.38 x 10-6 S/cm was evaluated for 20 wt% loading of BaCl2 in polymer electrolyte as shown in Fig As the loading wt% of inorganic compound increases, the number of carrier ions and Tg also increases. The optimum conductivity is probably due to the effective intercalation between charge carriers and polymer. The increased loading wt% of BaCl2 affects the chain mobility on ionic conductivity which can also be correlated with free volume of the polymer. Thus, 107

18 the main purpose of adding BaCl2 is to increase the free volume of polymer electrolytes resulting in an increase in ionic conductivity. Fig 5.8: Salt concentration dependence ionic conductivity for PSSA/PVA/BaCl2 polymer electrolyte at different temperatures. The ionic conduction may also occur as a function of temperature due to hopping process. The Arrhenius plot of pure and BaCl2 based polymer electrolyte is shown in Fig The temperature variation of conductivity of polymer electrolyte after addition of BaCl2 is shown in Fig. 5.9 (a-d). The liner graph has been obtained after fitting the entire data using liner fitting in origin. The evaluated result clearly defines that change in conductivity after addition of BaCl2. Hence, the BaCl2 plays a vital role to increase the conductivity of present polymer electrolyte. It is also observed that the ionic conductivity of polymer electrolyte increases as a function of temperature. This can also be defined as the free rationalized model (Rajendran et al., 2007; Chandra et al., 2009). At higher temperature, thermal movement of polymer chain segments and the dissociation of salt would be improved, which increase the ionic conductivity. Therefore, as the temperature increases the free volume increases in polymer electrolyte. The mobility of ions may also be described by evaluated activation energy as tabulated in Table 5.4. The evaluated activation energy for pure PSSA and PVA is and ev respectively. Whereas, 108

19 the activation energy gradually decreases in addition of BaCl2 this is more significant for high conductivity. It represents that the activation energy decreases at higher loading wt% of BaCl2 in polymer electrolytes. However, at lower temperature presence of BaCl2 leads to salt polymer or cation-dipole interaction, which increases the cohesive energy of polymer network. Hence, for lower temperature the free volume was decreased and because of less ionic mobility the ionic conductivity decreased. Fig 5.9: Temperature dependant ionic conductivity of PSSA/PVA/BaCl2 for (a) 45:50:5, (b) 40:50:10, (c) 35:50:15 and (d) 30:50:20 loading wt%. Table 5.4: Activation energy from Arrhenius plots of pure PSSA and PSSA/PVA/BaCl2 for (a) 45:50:5, (b) 40:50:10, (c) 35:50:15 and (d) 30:50:20 at different temperature. Sample Code Activation Energy (ev) Pure PSSA a 0.26 b 0.24 c 0.23 d

20 5.4.8 DETERMINATION OF DIFFUSION COEFFICIENT AND MOBILITY CHARGE CONCENTRATION The mobility of cations and anions in polymer electrolyte system can be determined by term diffusion coefficient. It is difficult to quantify the exact amount of mobility charge carrier in polymer system, because most of them are bounded with ion pairs or clusters. The Trukhan model is the most appropriate method to determine the total charge carrier concentration in polymer electrolyte system (Sorensen and Compan, 1995). The Trukhan model basically describes the mobility of charge carriers and helps to determine the diffusion coefficient and mobility charge concentration with respect to loss tangent (tan ) values (Munar et al., 2011). The diffusion coefficient depends on cation and anions symbolized by D+ and D- term. The Trukhan model is formularized as following: =... tan The diffusion coefficient equation as shown above is depends on maximum of tan spectra, the sample thickness L and the frequency value at which the tan is maximum. By substituting a maximum value in above equation, the diffusion coefficient was evaluated. Fig represents the diffusion coefficient as a function of temperature for activation energy Ea=0.26 and 0.22 ev, which is clearly of the Arrhenius type. The optimized response of diffusion coefficient as a function of temperature defines the semicrystalline nature of polymer electrolyte. This could be due the effect of ions presents in BaCl2. The obtained activation energy for semi-crystalline phase is high which is due to hopping of ions and polymer segmental motion (Karan et al., 2008). The ionic conductivity can also be correlated to the ion-ion interaction inside the polymer composite system. The mobility of ions demonstrates the behavior of present polymer electrolyte. The total ion concentration is represented by n and can be evaluated by following equation: =. Where, e is the elementary charge, D is diffusion coefficient, is an ionic conductivity, k is Boltzmann constant and T is absolute temperature. The inset Fig shows the 110

21 estimated free ion concentration as a function of temperature. The evaluated ion concentration is decreasing up to 40oC and then again starts increasing. The temperature at which the ion concentration is fairly constant can be defined as fusion temperature (40 o C). The changing trend of ion concentration as a function of temperature could be due to the effect of dissolved ions trapped in polymer semi-crystalline matrix. Fig 5.10: Arrhenius plot for the diffusion coefficient, computed according to Trukhan model. Ea=0.26 ev for 5 wt% and Ea= 0.22 for 20 wt% loading of BaCl2 in PSSA/PVA polymer electrolyte. The inset graph represents the estimated ionic concentration as a function of temperature The transference numbers corresponding to ionic (tion) and electronic (tele) transport have been evaluated of the PSSA/PVA/BaCl2 polymer electrolyte system using Wagner polarization technique (Wagner and Wagner, 1957). The transference number (tion and tele) are calculated using the relation = = 111

22 Where, Ii is initial current and If is the final residual current. The resulting transference number for different composition of PSSA/PVA/BaCl2 polymer electrolyte is shown in Table 5.5. Table 5.5: Transference number of PSSA/PVA/BaCl2 polymer electrolytes for different concentration. Polymer electrolyte (wt%) Transference Number tion tele PSSA/PVA/BaCl2 (40:50:10) PSSA/PVA/BaCl2 (35:50:15) PSSA/PVA/BaCl2 (30:50:20) The transference number is measured to be which indicates that the conduction in electrolyte is predominantly ionic and only a negligible contribution comes from electrons. A very slight change in transference number upon ion irradiation confirms that the ion irradiation does not change the conduction mechanism (SubbaReddy et al., 2003). 5.5 CONCLUSION The conclusion of overall results obtained from different characterization techniques implement on PSSA-PVA-BaCl2 polymer electrolyte are as follows: Polymer electrolyte consisting of PSSA, PVA and BaCl2.2H2O were developed and characterized. The structural, thermal and electrical analysis revealed the loading of BaCl2 in PSSA/PVA polymer system had a positive effect on ionic motion as well as phase structure of polymer. The optical band gap values evaluated from UV shows the increasing trend with an increasing loading wt% of BaCl2 in polymer electrolyte. Result of optical band gap measurement from UV spectrum indicates the presence of π π* transition by increase in Eg values as function of BaCl2 loading in polymer system. 112

23 DSC analyses highlighted the immiscibility and crystalline nature of polymer electrolyte. The DSC results were also confirmed by the XRD results. The ionic conductivity enhancement observed in PSSA/PVA/BaCl2 system evidently results the effect of compound. The ionic conductivity depends on salt concentration increases with loading wt% of BaCl2. The highest room temperature conductivity 9.38 x 10-6 S/cm was evaluated for 20 wt% loading of BaCl2 in polymer electrolyte. The Arrhenius relationship which shows hopping of ions in polymer composites describes the temperature dependence ionic conductivity of the polymer electrolyte. The diffusion coefficient parameter for barium chloride salt dissolved in polymer electrolyte which describes the movement of cation and anion is determined by loss tangent spectra. The transference number and enhanced conductivity represents that the charge transportation is dues to ions. Therefore this polymer electrolyte can be further used for the development of electrochemical cells with respect to its charging discharging capacity. 113