D. Selvakumar, P. Nagaraju, R. Jayavel
Keywords: graphene, nanocomposite, supercapacitors, energy storage
Summary:Two dimensional graphene-based nanocomposites have been synthesized through the modified Hummer’s method and chemical reduction process. Transition metal oxides were decorated on the graphene sheets by in-situ hydrothermal method. The composites were formed with graphene oxide and various metal oxides like SnO2, CeO2, MoS2, TiO2, RuO2 and V2O5 by homogeneous co-precipitation method. Multi-layered composite structures with 1 D carbon nanotubes integrated with 2 D graphene structures with the addition of 3 D bulk nanoparticles have also been fabricated. As-synthesized graphene showed the wrinkled paper like morphology as revealed by SEM/HRTEM studies. The prepared composite structure has been subjected to structural, optical, electrical property studies. The structure and phase formation were confirmed by XRD analysis. The reduction of graphene oxide to graphene was ascertained by the FTIR spectral analysis. The prepared composite structures exhibit enhanced thermal stability compared to pure metal oxide, graphene oxide and graphene. The Raman spectral analysis was performed to confirm the coordination of the compounds through the G and D band peaks along with the overtones. X-ray photoelectron spectroscopy revealed the presence of all constituents and the oxidation states of the compounds. The surface area of the composite structures was compared and the maximum value of 250 m2g-1 was obtained for MoS2 based composite. The electrochemical properties of graphene-metal oxide composites reveal that these materials can be effectively used as electrodes for supercapacitor application with improved specific capacitance, higher power density and cyclic stability. Highest specific capacitance value of 850 F/g-1 was achieved for RuO2 and MoS2 metal oxides based composites. The cyclic voltammetry studies reveal the current density in the ampere range. The energy density of the composite structures were comparable with that of conventional batteries and the power density comparable to the capacitors. The cyclic stability of the composite electrode was tested up to 5000 cycles with the capacitance retention of more than 95 %. The Coloumbic efficiency and the specific capacitance were also estimated from the charge-discharge curves recorded through chronopotentiometry. Flexible free standing reduced graphene oxide (rGO) papers of dimension 10x10 cm2 have been fabricated by evaporation induced self assembly process and their electrical, mechanical, optical and electrochemical properties have also been studied. The flexible rGO papers were used to fabricate several functional devices like bimorphic actuators and solid state flexible supercapactors. With enhanced electrochemical properties and excellent cyclic stability the graphene-metal oxide based composite electrodes are promising candidates for industrial scale energy storage applications.