Conductivity and thermal behavior of proton conducting polymer electrolyte based on poly (N-vinyl pyrrolidone)
Introduction
Solid polymer electrolytes are an interesting class of materials that offer significant technological application in the development of solid-state electrochemical devices. Special interest today is focused on polymer systems having high ionic conductivity at ambient temperature since they find unique applications as electrolytes in electrochromic windows, super capacitors, fuel cells etc., [1], [2], [3]. In designing a novel polymer electrolyte, emphasis is placed on increasing the amorphous content in the polymer, which assists fast ion motion while maintaining its mechanical stability [4]. The percentage of amorphous nature of the polymer can be increased by the addition of salts to the polymer matrix. It has been reported that the electrical and optical properties of the polymers can be controlled by the appropriate dispersion of salts in the polymeric matrix [5]. Proton conducting polymer electrolytes could be prepared by complexation of various inorganic acids. There are several reports concerning the effect of addition of inorganic acids on the properties of the polymer electrolytes [6], [7]. However, the mechanical and chemical stability of these complexes with inorganic acids are relatively poor and chemical degradation is observed after humidification [8]. Apart from inorganic acids based proton conducting polymer electrolytes, polymer electrolytes having ammonium salts as dopants based on host polymers such as, polyvinyl alcohol [9], poly acrylamide [10], poly (N-vinyl pyrrolidone) [11] etc., have also been reported. Among these polymers, poly (N-vinyl pyrrolidone) is a biocompatible and water-soluble polymer and has been frequently investigated classes of materials for use in medicine and in other applications interfacing with biological systems [12]. PVP is an amorphous polymer possessing high Tg (359 K) due to the presence of the rigid pyrrolidone group, which is known to form various complexes with many inorganic salts [13], [14]. In the present work, ammonium thiocyanate (NH4SCN) doped PVP based polymer electrolytes have been characterized using XRD and ac impedance spectroscopic measurements and the thermal properties of the polymer electrolytes has been characterized using differential scanning calorimetry (DSC).
Section snippets
Experimental details
The polymer electrolytes, PVP (Mw = 40 000, SISCO) with NH4SCN (SISCO) in the molar ratios (95:05), (90:10), (85:15), (80:20) and (75:25) were prepared by solution casting technique using distilled water as a solvent. The X-ray diffraction pattern of the polymer electrolytes has been taken using Philips X-ray diffractometer PW 1830 with Cu Kα radiation. Differential scanning calorimetry (DSC) thermo-grams were obtained using Perkin–Elmer system DSC7 at a heating rate of 5 K/min under nitrogen
XRD analysis
Fig. 1(a)–(c) shows the XRD pattern of pure PVP and PVP doped with NH4SCN having different mole ratios. It is observed from Fig. 1(a), a broad peaks between 12°–15° and 19°–24° can be associated with the amorphous nature of pure PVP [15]. The relative intensity of the broad peaks between 12°–15° and 19°–24° decrease with increase of NH4SCN concentration. This may be due to the decrease of crystalline fraction of poly (N-vinyl pyrrolidone). This result can be interpreted by considering the Hodge
Conclusion
The electrical conductivity study reveals that the bulk conductivity of the electrolytes increases in the order of 10−7–10−4 S cm−1 as the concentration of ammonium thiocyanate increases from 5 to 20 mol%. The XRD spectrum confirms the amorphous nature of the polymer electrolytes. Thermal analysis indicates that the glass transition temperature and the degree of crystallinity is low for the 80 PVP:20 NH4SCN polymer electrolyte. The maximum conductivity has been found to be 1.7 × 10−4 S cm−1 for 20 mol%
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