Herein, we deliver a brief discussion on the classification, state-of-the-art progress, challenges, and perceptions of the redox-additive materials in the aqueous, nonaqueous, and solid-state electrolytes for high-performance supercapacitors. For the performance of electrochemical capacitors, electrolytes are found to be influential components, governing vital parameters including the voltage window, power, and energy density. To improve the electrolyte performance, the inclusion of redox additive species is counted as the best method where the redox reaction that occurs at the electrode−electrolyte interface mainly contributes to the overall enhancement of the device in terms of energy and power as well as stability. The method of preparation and utilization is quite simple, safe, and cost-effective in comparison with some active electrode materials. Hence, the chemistry behind redox additives seems to be of special interest, and the identification of novel redox additives is believed to be a hotspot in the area of supercapacitor electrode materials. In this, we focus on the interaction between carbon-based electrode materials in different redox-additive electrolytes and their challenges and propose different perspectives that concisely intend to enhance energy density without compromising other merits that are inherent.

The energy storage performances of supercapacitors are expected to be enhanced by the use of nanostructured hierarchically micro/mesoporous hollow carbon materials based on their ultra-high specific surface areas and rapid diffusion of electrolyte ions through the interconnected channels of their mesoporous structures. In this work, we report the electrochemical supercapacitance properties of hollow carbon spheres prepared by high-temperature carbonization of self-assembled fullerene-ethylenediamine hollow spheres (FE-HS). FE-HS, having an average external diameter of 290 nm, an internal diameter of 65 nm, and a wall thickness of 225 nm, were prepared by using the dynamic liquid-liquid interfacial precipitation (DLLIP) method at ambient conditions of temperature and pressure. High temperature carbonization (at 700, 900, and 1100 °C) of the FE-HS yielded nanoporous (micro/mesoporous) hollow carbon spheres with large surface areas (612 to 1616 m2 g−1) and large pore volumes (0.925 to 1.346 cm3 g−1) dependent on the temperature applied. The sample obtained by carbonization of FE-HS at 900 °C (FE-HS_900) displayed optimum surface area and exhibited remarkable electrochemical electrical double-layer capacitance properties in aq. 1 M sulfuric acid due to its well-developed porosity, interconnected pore structure, and large surface area. For a three-electrode cell setup, a specific capacitance of 293 F g−1 at a 1 A g−1 current density, which is approximately 4 times greater than the specific capacitance of the starting material, FE-HS. The symmetric supercapacitor cell was assembled using FE-HS_900 and attained 164 F g−1 at 1 A g−1 with sustained 50% capacitance at 10 A g−1 accompanied by 96% cycle life and 98% coulombic efficiency after 10,000 consecutive charge/discharge cycles. The results demonstrate the excellent potential of these fullerene assemblies in the fabrication of nanoporous carbon materials with the extensive surface areas required for high-performance energy storage supercapacitor applications.

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