Polymer composites based on functionalized metal analogues incorporated with ionic liquids for electrochemical applications

Abstract

A series of polymer composites have been prepared by using both conducting and non-conducting polymer as matrix phases and different metal oxides as dispersed phases. Conducting polymer, polyaniline (PAni) was prepared by chemical oxidative polymerization as well as electrochemical polymerization while simple solution casting method was used for commercially available non-conducting polymer, poly(vinyl alcohol) (PVA). PAni based MnO2 and NiO composites and PVA based MnO2 composites were prepared with a view to achieving novel functionalities as well as for their electrochemical application in supercapacitors. Compatible ionic liquids (ILs) were also incorporated into the polymer composites to monitor influence on the capacitive properties. For instance, hydrophobic IL, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulphonyl) imide, [C2mim][TFSI] and hydrophilic IL, 1-ethyl-3-methylimidazolium tetrafluoroborate, [C2mim][BF4] were incorporated into the chemically prepared PAni-MnO2 and PVA-MnO2 composites, respectively and their capacitive behavior were critically examined. PAni-MnO2 composites with varying wt.% of MnO2 were prepared by in situ chemical oxidative polymerization using MnSO4 dissolved in aqueous acidic solution of aniline monomer followed by the addition of KMnO4 in which MnO2 was formed and simultaneously acted as an oxidant to initiate the polymerization. For optical, structural and morphological characterization of the composites UV-visible absorption and specular reflectance spectra, Fourier transform infrared (FT-IR) spectra, X-ray diffraction (XRD) and scanning electron microscopic (SEM) images were used. Thermogravimetric analysis (TGA) was carried out to study the thermal stability and verification of wt.% ratio of PAni and MnO2 of the composites. X-ray diffraction analysis was performed to evaluate the crystallinity and phase purity and BET surface analysis was carried out to find out the specific surface area and porosity of the composites. Graphite electrodes were modified with the prepared composites by solvent casting method using dimethylsulfoxide and ethanol as solvents. [C2mim][TFSI was also incorporated into the composites. The electrochemical properties of the modified electrodes were analyzed by cyclic voltammetry, chronopotentiometry, and electrochemical impedance spectroscopic (EIS) techniques. The specific capacitance was determined from the data obtained from chronopotentiometric measurements of the prepared composites and the results were analyzed for their applications in supercapacitors. PAni-NiO nanocomposites with wt.% variation of NiO NPs were prepared by in situ chemical oxidative polymerization using aniline monomer and pre-formed NiO NPs served as the dispersed phase. Composites prepared were characterized to identify the properties and interaction among the components of the composites. TGA showed that thermal stability of PAni-NiO nanocomposites was higher than that of PAni. AC electrochemical impedance spectroscopic technique was employed and the data obtained were fitted to equivalent circuits. The resistance of the composites from the corresponding equivalent circuits was used to determine the conductivity of the composites. The band gap energy (Eg) of the composites was determined from the specular reflectance spectroscopic data by Kubelka-Munk method using Tauc equation. Dielectric constant and dielectric loss factor were determined from the impedance spectroscopic data. The value of dielectric constant and dielectric loss factor was evaluated for their applications as dielectrics. PAni and PAni-NiO composites were also prepared by electrochemical oxidative polymerization by cyclic voltammetry using aniline monomer and Ni2+ salt solution in acidic medium. The electrochemical performance of the electrochemically modified graphite electrodes with PAni and PAni-NiO composites was determined by cyclic voltammetry, chronopotentiometry and electrochemical impedance spectroscopic techniques. The specific capacitance was determined from the data obtained from chronopotentiometric measurements of the prepared composites and the results were analyzed for their applications in supercapacitors. PVA-MnO2 nanocomposites were prepared from by adding pre-formed MnO2 NPs in aqueous solution previously prepared by the reduction of MnSO4 with Na2S2O3 in aqueous medium to the aqueous solution of PVA. The composites were then solution casted and dried in an oven to obtain PVA-MNO2 nanocomposite film. Thermal behavior of PVA-MnO2 nanocomposite films changes with variation of MnO2 NPs loading. The degradation of PVA-MnO2 nanocomposite has been found to occur at lower temperature than that of PVA on addition of certain wt.% MnO2 NPs. The degradation temperature then remains constant up to a certain loading followed by lowering of degradation temperature with further addition of MnO2 into PVA. A complete shielding of UV radiation takes place in PVA-MnO2 nanocomposite films containing greater than 1.0 wt.% of MnO2 NPs. Supercapacitive performance of PVA-MnO2 nanocomposites modified graphite electrode with incorporation of a hydrophilic ionic liquid, [C2mim][BF4], in 0.5 M aqueous Na2SO4 solution was investigated. The results of specific capacitance of PVA-MnO2 and [C2mim][BF4] incorporated PVA-MnO2 nanocomposites are indicative of their feasibility for application in supercapacitors. PAni-NiO nanocomposites and PVA-MnO2 nanocomposite films exhibited novel functional properties. The dielectric constant and dielectric loss factor of PAni-NiO determined were significant to be used as dielectric materials. UV-visible spectra of PVA-MnO2 nanocomposite films in transmittance mode showed almost hundred percent absorption of UV radiation at very low loading of MnO2 NPs to indicate that PVA-MnO2 nanocomposite films have potential to be used as UV-shielding materials. Capacitive properties of MnO2 embedded conducting PAni and non-conducting PVA based composites have been compared to reveal the influence of the nature of polymer matrix and variation of MnO2 content of the composites. An attempt has also been made to evaluate the comparative capacitive properties upon incorporation of ILs into these polymer composites. Thus, a systematic and comprehensive analysis of the composites has been performed to assess various aspects of polymer composites with both conducting and non-conducting matrix phases for desirable chemistry which eventually open up new routes for applications of the composites as materials for UV-shielding and supercapacitors.