Sodium hexafluorophosphate mediated enhancement of electrical and electrochemical properties of poly(vinyl alcohol)-chitosan solid polymer electrolytes for EDLCs

A free-standing, flexible and biodegradable biopolymer electrolyte (BPE) derived from a poly(vinyl alcohol) (PVA)-chitosan (CS) blend immobilizing sodium hexafluorophosphate (NaPF₆) salt was fabricated via solution casting method. The effect of salt concentration on the structural, electrical, and electrochemical properties of the electrolyte was systematically investigated. X-ray diffraction (XRD) and Fourier-transform infrared (FTIR) spectroscopy were used to ascertain the microstructural changes in the polymer matrix including the complexation of PVA, CS, and NaPF₆. Electrochemical impedance spectroscopy (EIS) measurements revealed that the BPE containing 40 wt% NaPF₆ exhibited the highest conductivity (6.94 ± 0.04) × 10⁻⁵ S cm⁻¹, which was three-order enhancement over the pristine system. The ion transport behaviour, interpreted through the Schütt and Gerdes (S-G) model, revealed that the ionic conductivity of the SPE system is strongly influenced by both the concentration of charge carriers and their mobility. The electrolyte displayed a predominant ionic nature with an electrochemical stability window of ∼3.25 V. When incorporated into an Na-ion EDLC, the optimized electrolyte sample provided a specific capacitance of 42.65 F g⁻¹, energy density of 5.4 W h kg⁻¹, and power density of 95 W kg⁻¹, as determined by galvanostatic charge-discharge (GCD) tests performed at 0.05