Effect of Anhydrous Gdcl3 Doping on the Structural, Optical and Electrical Properties of PVP Polymer Electrolyte Films

Polymer electrolyte films containing GdCl3 salt and polyvinyl pyrrolidone polymer have been prepared by solution cast technique. X-ray diffraction patterns confirm the amorphous nature of all the films. UV-Visible optical absorption properties reveal the complex formation of polymer electrolyte with GdCl3 dopant. The absorption coefficient, direct band gap energy and indirect band gap energy are decreased with increasing GdCl3 dopant. Both dielectric and ac conductivity studies exhibit that the dielectric constant value at low frequency and ac conductivity at high frequency increases with increasing GdCl3 dopant concentration in PVP polymer. The complex impedance plots showed a single semi-circular arc for all the concentrations of CdCl3 doped in PVP. The ac conductivity and ionic conductivity are about 3.9610-3 S/cm and 3.1210-3 S/cm at room temperature for 15 mol% GdCl3 doped PVP polymer electrolyte films. The cyclic-voltammetry plots revealed large specific capacities with increasing in GdCl3 dopant concentration. The estimated ionic conductivity is five orders of magnitude larger than that of PVP doped with other metal salts reported earlier. The present study strongly recommends that the CdCl3 is worthy candidate for enhancing ionic conductivity of PVP and other polymers.


Introduction
Electrolyte films containing metal ions and conducting polymers have been known for their potential applications in ultra-capacitor, sensors and fuel cells. Generally, polymer electrolytes would exhibit two phases such as crystalline and amorphous phases. When these two phases are appeared in polymer electrolyte, the mechanism of ionic conductivity on polymer electrolyte films cannot be easily understood. The enhancement of ionic conductivity strongly depends on the amorphous nature of polymer electrolytes. The simultaneous movement of ions and the interaction of cation with anion also decrease the ionic conductivity of materials. Many researchers have suggested that the addition of a suitable metal salt in polymer network would drastically improve the charge transport which 2 leads to enhance the ionic conductivity of polymer electrolyte films. Till now, different polymers such as polyvinyl alcohol, polyvinyl chloride, polypropylene glycol, poly vinyl pyrrolidone, polyethylene oxide and polyvinylidene fluoride have been utilized to develop polymer electrolytes with high ionic conductivity at room temperature [1][2][3][4][5][6][7][8][9][10].
Among the polymer, poly vinyl pyrrolidone (PVP) have been chosen for its attractive properties such as the amorphous polymer, excellent film-formation with various salts, presence of carbonyl group (C=O), etc. The faster ionic mobility can be obtained in this type of polymer. The PVP polymer is worthy candidate for making polymer electrolyte films with doping of different metal ions due to its good mechanical and thermal stability, easy soluble in water and improved ionic conductivity. The significant improvement in polymer electrolyte film can be achieved by adding alkali metal salts in polymer electrolytes. Similar reports have already been reported earlier [2][3][4][5][6][7][8][9]. Apart from alkali metal salt-based polymer electrolytes, other metal salts such as divalent and trivalent metal salts were also used as the dopants in different polymers and polymer blends with the intention of enhancing the ionic conductivity but till now PVP doped with anhydrous GdCl3 have not been reported [10][11][12][13][14].
In this paper, the structural, optical and electrical properties of PVP doped with various concentrations of anhydrous GdCl3 have been discussed in detail. The 5, 10 and 15 mol% anhydrous GdCl3 doped PVP polymer electrolyte films have been prepared by simple solution casting method. Gd 3+ trivalent metal ion was chosen as the dopant for reporting the significant changes in the charge transport of PVP polymer. To best of our knowledge, this is first report on the effect of GdCl3 dopant on the ionic conductivity of PVP polymer electrolyte films.

Preparation of GdCl3 doped PVP polymer electrolyte films
Thick films of anhydrous GdCl3 doped PVP polymer electrolyte films were prepared using solution cast technique. An aqueous solution of PVP dissolved in double distilled water was continuously stirred at 70C until the homogeneous solution is obtained. It is named as PVP stock solution. The 5 mol% of GdCl3 dissolved in doubly distilled water was constantly stirred at room temperature for preparing GdCl3 aqueous solution and then this solution was mixed with the stock solution of PVP. Then the mixture solution of GdCl3 and PVP was continuously stirred for 2 h at room temperature. The resultant solution was poured on a cleaned glass plate and then kept into hot air oven to dry the solution at 70C for 1 hr. After drying, the thick film was peeled off from the plate and kept in vacuum desiccators. Similarly, 10 and 15 mol% GdCl3 doped PVP polymer electrolyte films were prepared.  Fig. 1 shows XRD plots of 5, 10 and 15 mol% GdCl3 doped PVP polymer electrolyte films. In Fig. 1, pure PVP shows a peak in the diffraction angle of 2 at 21, representing the semi crystalline of PVP [15]. XRD patterns of GdCl3 doped PVP polymer electrolyte films exhibited that the increase in the GdCl3 dopant concentrations in the PVP leads to decrease in the intensity which results the decrease in crystallinity of GdCl3 doped polymer electrolyte films. It is clearly well agreed with the report of Hodge et al [16]. Addition of GdCl3 to PVP polymer improves the amorphous phase of GdCl3 doped PVP films with increasing GdCl3 dopant concentration. It can be seen from Fig. 1 that no sharp peak was found for 15 mol% GdCl3 in the PVP, indicating complete dissociation of GdCl3 metal salt into the polymer which confirms the dominant role of amorphous phase in the PVP-GdCl3 salt complexes and results the more diffusion of ions from higher concentration to lower concentration of segments with high ionic conductivity [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16].

Optical absorption studies
UV-Visible optical absorption plot of pure PVP, 5, 10 and 15 mol% GdCl3 doped PVP polymer electrolyte films is illustrated in Fig. 2a. From the figure, it has been clearly observed that pure PVP polymer film does not show any absorption peak whereas GdCl3 doped PVP polymer electrolyte film exhibit a broad absorption peak. An addition of Gd 3+ ions in the PVP film causes the broad absorbance peak in the optical absorption spectra of GdCl3 doped PVP film. The absorption coefficient () can be determined by the following relation: where x is the film thickness, I is incident intensity, I0 is transmitted radiation and A corresponds to log(I/I0). Fig.   2b shows a plot between the absorption coefficient and photon energy. The absorption coefficient () can be estimated by the straight line fitting on linear portion of the curve to zero absorption and is 5.22, 4.96, 4.87 and 4.72 eV for pure PVP, 5, 10 and 15 mol% GdCl3 doped PVP films. Both direct band gap energy and indirect band gap energy can be calculated from the plot of (h) 2 vs h and (h) 1/2 vs h using the following equations [17,18].
where h is the optical energy, Egd is the direct band gap energy, Egi are indirect band gap energy of material, 1 is the constant for direct transition and 2 is the constant for indirect transition [17][18]. The direct band gap energy  frequency increases with the rise in temperature. When the temperature is increased, the dipoles formed in the polymer electrolyte films has gained a sufficient kinetic energy and are orientated themselves which causes the enhancement in the dielectric constant [3,4]. The dielectric constant decreases with increasing frequency and remains constant at high frequency side. This behavior is due to the ability of the dipoles to rotate themselves in the direction of applied electric field between electrolyte and electrode at their respective electrodes whereas the rotation of dipoles is lagging behind the applied electric field and thereby dielectric constant decreases at high frequencies [6].

AC conductivity analysis
AC conductivity plots of pure PVP and GdCl3 doped PVP films are shown in Fig. 4. The ac conductivity increases with increasing of GdCl3 concentration. It confirms that the mobility of anions has been completely stopped and leads to increase in the conductivity of polymer-salt complexes by the addition of Gd 3+ cations in PVP polymer electrolyte film. It is worthy to note that the ac conductivity of GdCl3 doped polymer electrolyte films is larger than those of obtained in pure PVP film. The highest value of conductivity is about 3.9610 -3 S/cm at room temperature for 15 mol% GdCl3 doped PVP film. The highest value of ac conductivity also attributes to the reduction in glass transition temperature which gradually reduces the crystallinity of the PVP polymer film with doping of GdCl3 as discussed in XRD studies [6,8]. A frequency independent region at low frequencies was observed in ac conductivity plot and is associated to the dc conductivity while dispersion of dipoles at high frequencies was observed and is associated to the ac conductivity. The results obeyed the Jonscher power law which is given below: n       (1) where σ0, A and n are known as the dc conductivity, pre-exponential factor and the fractional exponent between 0 and 1. The value of n can be obtained by fitting Jonscher power law using experimental data and is varying from 0.78 to 0.98. The obtained n value lies between 1 and 0.5 reported as the standard value of an ideal electrolyte [6,8,19]. Fig. 5 shows the ac conductivity of 15 mol% GdCl3 doped PVP polymer electrolyte films at different temperatures. As the temperature increases, the conductivity at high frequencies also increases which is due to the enhancement in the mobility of charge carriers [19].  [14,26]. The enhancement in dc with increasing in GdCl3 salt might be attributed to dissolution of GdCl3 salt in PVP with strong amorphous nature and boost the number of mobile charge carriers. A high dispersion can be obtained due to the strong interaction of PVP with GdCl3 salt, thus the conductivity increased. Fig. 7 shows the temperature dependent complex impedance plots of 15 mol% GdCl3 doped PVP film. A depressed semicircle along with inclined spike can be observed in the Fig. 7. The depressed semicircle represents the non-Debye nature of the 15 mol% GdCl3 doped PVP polymer electrolyte film whereas the inclined spike can be associated to electrode surface polarization [3]. The inclined spike also attributes to the formation of double layer capacitance at the electrode-electrolyte interface due to movement of ions from one site to another side [12]. The diameter of the semicircular arc decreases with increasing frequency and temperature. Then the semicircular arc is disappeared in plot which shows only a spike above 363 K.
The disappearance of semicircle at high temperature indicates the movement of Gd 3+ ions to contribute the conduction [21,22]. To find the activation energy, the experimental data was fitted by the following Arrhenius exponential law: where 0 is the pre-exponential factor, Ea is the activation energy, k is the Boltzmann constant and T is the absolute temperature [6]. Fig. 8 shows a plot of log dc vs 1000/T (K -1 ). The activation energy estimated from the Fig. 8 by fitting the Arrhenius equation mentioned above to the experimental data and is about 0.118 eV which is smaller than those previously reported for polymer electrolyte films containing poly vinyl alcohol and LiFePO4 salt [20]. The minimum Ea value is required for overcoming the potential barrier in the electrolyte films. Fig. 9a shows the real (M′) part of room temperature electric modulus for pure PVP and GdCl3 doped PVP polymer electrolyte films. In the Fig. 9a, the M′ at low frequency is very small due to the fact that electrode polarization is removed [8]. It can be also seen from the figure that the M′ value is decreased with increasing of the GdCl3 concentrations. Fig. 9b shows the imaginary (M′) part of electric modulus for all the films at room

Electrical modulus formalism
temperature. An asymmetric peak has been observed in the imaginary part of electric modulus plot. The asymmetric peak is shifted towards the region of high frequency, indicating a decrement in relaxation time and thereby enhances the conductivity. Fig. 10 shows the (a) M′ and (b) M′ plots for 15 mol% GdCl3 doped PVP polymer electrolyte film at different temperatures. In the Fig. 10a, the value of electric modulus decreases with increase in temperature. The value of M′ at high frequency side reveals the higher dispersion which tends to M due to the presence of conductivity relaxation. At low frequency side, M′ becomes to zero due to the fact that the value of electrode polarization is negligible [6,8]. Fig. 10b shows a peak in imaginary part of electric modulus plot. The peak was disappeared at 353 K and 363 K due to the limitation in frequency range. When the large amount of charge carriers is accumulated at the electrode-electrolyte interface, the M″ plot shows the smallest value at low frequency. Fig. 11 shows a plot of log fmax vs 1000/T for finding activation energy (Ea where 0, k and T are the pre-exponential factor, Boltzman constant and absolute temperature [27]. The activation energy calculated by the least square fitting to the experimental data is about 0.13 eV. Fig. 12

Cyclic Voltammetry studies
Fig . 13 shows the cyclic voltammetry curves for pure PVP and GdCl3 doped PVP polymer electrolyte films.
The cathodic and anodic peaks were appeared in the cell for 5, 10 and 15 mol% GdCl3 doped PVP polymer electrolyte fims, indicating that the GdCl3 dopant in the PVP polymer electrolyte films did not interact with aluminium electrodes. The large area of a curve can be observed in Fig. 13 and area of the curve increases with increases in the concentration of GdCl3 dopant. It represents the presence of higher specific charge than those observed in PVP film. It should be noted that the film undergoes a relatively broad reversible redox process, which illustrates its electroactivity. The cyclic voltammograms clearly shows the cyclability and reversibility of all of the electrolyte films [28,29].

Conclusion
Polymer suggest the PVP films doped with GdCl3 as the good candidates for electrochemical device applications.

Conflict of Interest
The authors declare that they have no conflict of interest.