Currently, to satisfy the power demand and reduce the world’s dependency on fossil fuels, the search for advanced energy storage technologies with enhanced safety has become a global objective in scientific research. Among energy storage systems, supercapacitors (SCs) have attracted significant research interest owing to their high-power density (~ hundred to many thousand times superior to batteries), superior charge/discharge performance, and long cycle life (>105 cycles). But the low energy density of the SCs (0.1 to 10 Wh kg-1, which is ~ 3-30 times less than batteries) push them out of competition in the market.1 However, the SCs with organic electrolytes generally offer high energy density. But toxicity, environmental unfriendliness, high fabrication cost, and poor conductivity of organic electrolytes greatly limit SCs on a commercial scale. Additionally, a high self-discharge/leakage current further reduces the power and energy outputs of the SCs.2 The present research work is comprising of four working chapters. Chapter-1 highlights the importance of mild acidic electrolytes over strong acidic electrolytes. In this work, to reduce the corrosive nature of strong acidic electrolyte (1 M H2SO4), a mixed electrolyte system (0.25 M H2SO4 + 1 M KNO3) is proposed for SC applications. But the low over-potentials for both oxygen and hydrogen evolution reactions in the proposed mild acidic medium is highly restricted the operating voltage of SC ~ 1 V and delivered an energy density of 6.94 Wh kg-1 at 1 A g-1. To further improvise the energy density of the aqueous SC, an alternative strategy is employed in Chapter-2. Since energy density has a quadratic dependence on voltage, broadening the operating voltage window of the aqueous SCs could be an effective strategy to boost the energy density, and this is achieved by choosing the right combination of functionalized electrode-electrolyte interface. This strategy resulted in a 2 V SC with an attractive energy density of 28 Wh kg-1 at 0.5 A g-1. Besides, the coupling of the energy-dense battery type electrode to the high-power dense SC electrode could boost the energy density along with high power output. The merits of integrating battery-type electrodes with high capacitance electrodes are highlighted in Chapter-3. In this chapter, a hybrid SC is developed by coupling the metallic Zn with highly porous activated carbon, and it delivered an extremely high energy density of 127 Wh kg-1 at 0.1 A g-1. The most striking feature of this hybrid SC is its low open-circuit voltage decay (34 % in 60 h) and low leakage current density (11 mA g-1), which allows it to hold the charge for a longer duration qualifying it as one of the best aqueous SC known in the literature. A proper choice of binder could result in high charge storage capacity without blocking pores in the active material. Chapter-4 emphasizes the development of high performance, biodegradable, environmentally benign, and cost-effective chitosan/poly(ethylene glycol)-ran-poly(propylene glycol) [Ch/poly(EG-ran-PG)] based polymer blend as membrane-cum-separator as well as a green binder for aqueous acidic SC applications as an alternative to Nafion®112 membrane as well as PVDF binder without compromising on the performance.
References
1. S. Wu et al., Adv. Energy Mater., 9, 1970184 (2019).
2. B. K. Kim, S. Sy, A. Yu, and J. Zhang, Handb. Clean Energy Syst., 1–25 (2015).