Enlarging energy density of supercapacitors using unequal graphene electrodes and ionic liquid electrolyte

Tuning oxygen-containing functional groups of graphene for supercapacitors with high stability

To investigate the relationship between the oxygen-containing functional groups of graphene and the stability of supercapacitors, reduced graphene oxide (rGO) containing different oxygenic functional groups was prepared by varying the reduction time of GO using hydrazine as the reducing agent. TEM, XRD, Raman, and XPS characterizations revealed that, as the reduction time increased, the sp² structure in the rGO sheet was restored and the obtained rGO had good crystallinity accompanied by removal of the oxygenic functional groups. The analysis of the content of the different functional groups also suggested that the reduction rate of the oxygenic functional group was C-O > C[double bond, length as m-dash]O > O-C[double bond, length as m-dash]O. The supercapacitive performance of rGO showed that the oxygenic functional groups contributed to some pseudocapacitance and resulted in a larger specific capacitance. At the same time, however, it is also accompanied by poorer rate performance and durability, which will be improved by removing the oxygenic functional groups by extending the reduction time. With an optimized reaction condition of a reduction time of 24 h, the obtained rGO exhibited excellent stability in floating tests at 3.0 V and 45 °C for 60 days. These findings pave the way for the development of high quality graphene materials for cost-effective and practical graphene supercapacitors.

Recent Advances on Boosting the Cell Voltage of Aqueous Supercapacitors

Due to its ultra-fast charge/discharge rate, long cyclic life span, and environmental benignity, aqueous supercapacitor (SC) is considered as a proper next-generation energy storage device. Unfortunately, limited by undesirable water electrolysis and unreasonable electrode potential range, aqueous SC normally generates a narrow cell voltage, resulting in a low energy density. To address such challenge, enormous efforts have been made to construct high-voltage aqueous SCs. Despite these achievements, the systematic reviews about this field are still rare. To fill this knowledge gap, this review summarizes the recent advances about boosting the cell voltage of aqueous SCs. From the viewpoint of electrode, doping alkali cations, modulating the electrode mass ratio, and optimizing the surface charge density are regarded as three effective pathways to achieve this goal. However, adjusting the appropriate pH level, introducing redox mediators, and constructing "water-in-salt" electrolyte are other three universal routes from the electrolyte aspect. Furthermore, it is also effective to obtain the high-voltage aqueous SCs through asymmetric design, such as designing asymmetric SCs. The confronting challenges and future development tendency towards the high-voltage aqueous SCs are further discussed.

Konjac Sponge Derived Carbon Flakes with Optimized Pore Structure for High-Performance Supercapacitor

Lamellar activated carbons derived from Konjac sponges (KACs) have been successfully fabricated through a facile KOH activation method. By manipulating the activation temperature and KOH/C ratio, the achieved KACs exhibit ultrahigh specific surface area up to ∼3000 m2/g and hierarchical pore structure with tunable micro/mesopore distribution. Notably, KACs possess plenty of worm-shaped micropores formed by graphene stacking layers with the lateral distance close to size of hydrated electrolyte ions. Owing to optimized pore structure, high graphitization, and extra O/N doping, KACs exhibited much enhanced specific capacitance (253.0 F/g), superior rate ability (77% retention of capacitance at 10 A/g), and remarkable cycling stability (0.4% decay under 5 A/g after 2000 cycles) in the acid electrolyte. The mass production ability of KAC materials and the knowledge of correlation between texture properties and capacitive performance open new opportunities for the application of such novel biomass-derived carbons in supercapacitor devices.