Synthesis, characterization and theoretical studies on novel organic–inorganic hybrid ion–gel polymer thin films from a γ-Fe2O3 doped polyvinylpyrrolidone–N-butylpyridinium tetrafluoroborate composite via intramolecular thermal polymerization

Abstract

Developing new varieties of highly ionic conducting polymer organic–inorganic hybrid ion–gel composites is important for improving the efficiency of energy storage devices to satisfy the current growing need for energy requirements. The scope of iron oxide doped polymer ion–gels in this field has not been investigated much. Therefore, herein, we propose a facile low cost synthesis and material characterization of a new type of flexible, hydrophilic organic–inorganic hybrid ion–gel polymer from polyvinylpyrrolidone (PVP), N-butylpyridinium tetrafluoroborate (bpy[BF₄]) ionic liquid (IL) and γ-Fe₂O₃ magnetic nanoparticles (MNPs) with improved specific capacitance and energy density. The formation of a bpy⁺˙ cation radical in the thermally stable 1 : 1 PVP–IL ion–gel matrix caused free radical induced thermal polymerisation of PVP chains at higher temperatures giving a flexible PVP–bpy[BF₄] ion–gel thin film which was ESR active before and after cross-polymerization. Studies revealed a proton mediated electron migration occurring from PVP to bpy[BF₄] causing a reduced rate of electron–hole recombination in the PVP–IL matrix. We have also theoretically probed the intermolecular interactions of PVP and IL by evaluating the PVP–IL binding energy, changes in enthalpy and entropy changes before and after binding, density of states (DOS) distribution and critical points (CP) analysis using Density Functional Theory (DFT). Doping a 1 : 6 wt% ratio of γ-Fe₂O₃ MNPs into the PVP–IL matrix increased the ionic conductivity of the thin films from 331.7 to 450.66 μS cm⁻¹ and the specific capacitance values from 2.575 and 92.215 F g⁻¹ respectively. The γ-Fe₂O₃ doped PVP–IL inorganic–organic hybrid thin film was found to be superior compared to the PVP–IL thin film. The high dielectric γ-Fe₂O₃ MNPs effectively reduced the band gap, electron–hole recombination rate, charge transfer resistance, solid state interface layer resistance and leakage current and also improved the charge–discharge profile and energy density of the newly synthesised PVP–IL ion–gel thin film.