SYNTHESIS OF N-DOPED REDUCED GRAPHENE OXIDE AND ITS NANOHYBRIDS AS ELECTRODE MATERIAL(S) FOR ELECTROCHEMICAL APPLICATIONS - AN ANALYSIS OF THE ROLE OF DIFFERENT AQUEOUS ELECTROLYTES ON THEIR SUPERCAPACITIVE PERFORMANCE

Abstract

The depletion of conventional fossil fuels and increasing demand of renewable energy resources has drawn global attention of scientific community for developing the efficient energy storage devices (batteries, supercapacitor and fuel cell). Among these, the supercapacitor has been recognized as one of the future energy storage device(s) due to its high power delivery capabilities, long cycle life and low charging time. Despite of these advantages, the major issue with the supercapacitor-based devices is its poor energy density, as the present used commercial supercapacitor has very low energy density (5-10 Wh/kg) as compared to popular energy storage device, the lithium-ion battery (100-260 Wh/kg). The energy density (Ed = 1/2 CV2) shows dependence on capacitance and square of operating potential window. Thus, for increasing the energy density of supercapacitor, one would require the appropriate electrode-electrolyte combination that can provide high capacitance and be electrochemically stable to operate in wide potential window. In this reference, a variety of carbon allotropes namely, activated carbon (ACs), carbon nanotubes (CNTs) and graphene based nanostructures have been explored extensively as the electrode material(s) for energy storage devices because of their novel surface and structural physiochemical characteristics. Among these, the graphene due to its exceptionally unique features containing sp2 hybridized carbon associated with relatively higher electrical conductivity (~106 S cm-1), specific surface area (~2630 m2 g-1), mechanical strength (42 N/m2) and its easy manipulation for achieving favorable pore size, i.e. consisting of both micro- and mesopores for accessing the electrolyte ions, makes it an attractive material for supercapacitor. But, restacking of pristine graphene sheets through van der Waals forces reduces its effective surface area in contact with the electrolyte ions, thus restricts its usage as an efficient electrode material. The functionalization/doping of graphene/reduced graphene oxide (rGO) with heteroatom(s) like B, N, P and S has been extensively explored to prevent the restacking as well as to simultaneously provide the pseudocapacitance. Specifically, the doping of N in the graphene/rGO nanostructure have been found to be more effective because of its size compatibility with C and fairly high electronegativity and electron density which could enhance the overall electrochemical performance. The electrochemical characteristics of N-doped rGO could further be enhanced by making its nanohybrids with certain metal oxide(s) by reducing the restacking and improving the pseudocapacitance. However, most of the previous studies on their syntheses have been performed in non-aqueous medium following two/three steps at relatively higher temperature.