IN SITU SYNTHESIS OF HETEROATOM(S) (N/S/P) CO-DOPED POROUS REDUCED GRAPHENE OXIDE AS ELECTRODE MATERIAL(S) FOR DESIGNING OF BINDER-FREE AND HIGH-MASS LOADED SUSTAINABLE HIGH PERFORMANCE AQUEOUS SYMMETRIC SUPERCAPACITOR AND ELECTROCHEMICAL SENSOR

Abstract

The depletion of traditional fossil fuels, like coal, oil and natural gas and increasing environmental concerns has spurred world-wide the scientific community to focus on renewable energy sources (solar/wind/hydro) and to develop efficient energy storage devices (such as batteries, supercapacitors, and fuel cells). In this context, the renewable energy production has expanded widely around the globe. But as regards to the energy storage tools, supercapacitors are envisaged as the promising device for future on account of their relatively longer cycling life and rapid charge and discharge rates without any need for frequent maintenance. It thus offers high-power density besides being cleaner. Owing to their small size and light weight, supercapacitors are now being used as power source for: short-term acceleration with enough capacity, temporary energy storage during braking, portable electric devices such as smartphones, laptops and notebooks, and hybrid electric vehicles. Despite of these advantages, a significant drawback of supercapacitor-based device(s) is their poor energy density. Currently, commercially available supercapacitors are relatively much inferior with regards to their energy densities (7 to 10 Wh/kg) as compared to those of popular energy storage devices like lithium-ion batteries (~300 Wh/kg) and are needed to be improved further significantly in this respect. The energy density (Ed = 1/2 CV2) in supercapacitor shows dependence on capacitance (C) and square of operating potential window (V). Therefore, for increasing their energy density, one would require an appropriate electrode-electrolyte combination that can provide high capacitance and electrochemically stable electrode material to operate in wide potential window. In this reference, a variety of carbon-based materials, specifically, low-dimensional carbon allotropes namely, carbon aerogels, activated carbon (ACs), carbon nanotubes (CNTs), and graphene-based nanostructures have been explored extensively as the electrode material(s) for electrochemical energy storage devices because of their novel surface and structural physiochemical characteristics. Among these, the pristine graphene due to its exceptionally unique features such as containing sp2 hybridized carbon associated with relatively higher electrical conductivity up to 80 mS/m, thermal conductivity of 5300 W/mk, specific surface area (~2630 m 2 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 the pristine graphene sheets, taking place through van der Waals forces reduces its effective surface area in contact with the electrolyte ions, thus restricts its usage as an electrode material for designing of an efficient supercapacitor. 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 provide simultaneously the pseudocapacitance. Further, their doping into the porous carbon structure is considered to be an important strategy to improve its electrical conductivity, surface area, creation of more defects/active sites to eventually increase its wettability and pseudocapacitance besides being environmentally benign. The impact of dopants on the specific capacitance of carbon-based materials is observed to depend on their chemical environment as well as the extent of doping, as the synergy between these two factors enhances the material performance. For example, N-5 and N-6, with negative charges, act as Faraday reaction sites, contributing to pseudo-capacitors, while positively charged quaternary N enhances electron transport in the carbon lattice. Additionally, the co-doping of heteroatoms, specifically, dual-doping/multi-heteroatoms doping into carbon framework is expected to further improve its performance: by modifying adjacent C atoms bonding and electron donor/acceptor abilities through the complex synergistic effect. It is also considered to be crucial for the modification of surface and electronic properties.