Energy storage is a crucial element of green transition that drives the growth of technology, and its importance is progressively increasing. The expanding demand for reliable power sources necessitates the development of innovative methods to balance energy production and consumption. Among energy storage devices (ESDs), supercapacitors are considered superior due to their outstanding power, swift charging, long lifespan, and sustainability. The ion mobilities in supercapacitors are mainly determined by the electrolyte, which further influences the charge transfer, voltage, and total power holding capacity. Due to the critical role of electrolytes in supercapacitors, several advancements are made in the design of the electrolyte to increase voltage strength and energy density. The electrolytes for supercapacitors include aqueous-based, organic solvent-based, ionic liquids (ILs) and solid-state electrolytes. Among them, IL-based electrolytes are the most suitable electrolytes due to their combination of beneficial physical and electrochemical properties. However, more than 95 % of the IL-based electrolytes are heavily based on fluorinated compounds, which are creating serious problems not only in the synthesis and implementation levels but also at the recycling stage. There is an urge to develop fluorine-free and non-flammable functional electrolytes for enabling next-generation supercapacitors.

This thesis is an attempt to elevate energy storage technology by providing economically and environmentally efficient solutions in terms of fluorine-free IL-based electrolytes for next-generation supercapacitors. The main aim is to design and identify Fluorine-Free Ionic Liquids (FFILs) as electrolytes for efficient next-generation supercapacitors. The first part of this thesis (Paper I) is focused on the synthesis, physical characterization, and transport as well as electrochemical properties of a novel class of ten FFILs derived from biomass. The biomass derived anions such as furan-2-carboxylate [FuA] and tetrahydrofuran-2-carboxylate [HFuA] are coupled to a range of nitrogen heterocyclic cations to create the FFILs, for which the nature of cation controlled their properties. The second part (Paper II) is dedicated to the synthesis and characterization of dialkylphosphate-based FFILs, that offered high thermal decomposition and low glass transition temperatures, and high ionic conductivities as well as high electrochemical stabilities. Further, their performance as electrolytes in symmetric supercapacitors showed high coulombic efficiency and capacity retention even after long charge-discharge cycles. The part three (Paper III) presents the synthesis, physical and electrochemical characterization of two novel fluorine-free zinc salts and their electrolytes derived from artificial sweeteners. The three-component zinc electrolytes are composed of either zinc saccharinate (Zn(Sac)2 or zinc acesulfamate Zn(Asf)2 salt, tetrabutylphosphonium saccharinate [P4444][Sac] IL, and γ-valarolactone (VL). These electrolytes demonstrated significantly long cycle life in Zn||Zn symmetric cells, while maintaining high capacity retention after extended cycles at a lower current density in an asymmetric hybrid supercapacitors (Zn||Ac).