Biomass-derived activated carbon materials were prepared by a two-step synthesis via carbonization followed by KOH activation of rice straw at 600 °C in an argon atmosphere. The formation of disordered micro- and mesopores on carbon by KOH chemical activation and the high specific surface area of ∼1007 m2 g–1 were confirmed by N2 adsorption–desorption. Further, the scanning electron microscopic analysis revealed the formation of disordered pores over the carbon surface, and the transmission electron microscopic analysis confirmed the formation and aggregation of ultrafine carbon nanoparticles of ∼5 nm in size after the carbonization and activation processes. The three-electrode cell in aqueous electrolyte shows high specific capacitance of 332 F g–1, with high specific capacitance retention of 99% after 5000 cycles. The fabricated symmetric supercapacitor device in aqueous 1 M H2SO4 electrolyte showed a high specific capacitance of 156 F g–1, with a high energy density of 7.8 Wh kg–1. The symmetric device fabricated using 1-ethyl-3-methyl imidazolium tetrafluoroborate ([EMIM]­[BF4]) ionic liquid exhibited a cell voltage of 2.5 V and a specific capacitance of 80 F g–1, with a high energy density of 17.4 Wh kg–1. The observed electrochemical performance clearly indicates that activated carbon derived from rice straw could be used as a promising electrode material in a supercapacitor for electrochemical energy storage. The cheaper and readily available rice straw raw materials, simple chemical activation process, and high performance promise that the obtained carbon material is viable for commercial applications in supercapacitors.

All-solid-state flexible asymmetric supercapacitors fabricated using CoS and AC showed a high cell voltage, high specific capacitance, and high energy density of 5.3 W h kg−1 with excellent electrochemical stability.

Materials which possess high specific capacitance in device configuration with low cost are essential for viable application in supercapacitors. Herein, a flexible high-energy supercapacitor device was fabricated using porous activated high-surface-area carbon derived from aloe leaf (Aloe vera) as a precursor. The A. vera derived activated carbon showed mesoporous nature with high specific surface area of ∼1890 m/g. A high specific capacitance of 410 and 306 F/g was achieved in three-electrode and symmetric two-electrode system configurations in aqueous electrolyte, respectively. The fabricated all-solid-state device showed a high specific capacitance of 244 F/g with an energy density of 8.6 Wh/kg. In an ionic liquid electrolyte, the fabricated device showed a high specific capacitance of 126 F/g and a wide potential window up to 3 V, which results in a high energy density of 40 Wh/kg. Furthermore, it was observed that the activation temperature has significant role in the electrochemical performance, as the activated sample at 700 °C showed best activity than the samples activated at 600 and 800 °C. The electron microscopic images (FE-SEM and HR-TEM) confirmed the formation of pores by the chemical activation. A fabricated supercapacitor device in ionic liquid with 3 V could power up a red LED for 30 min upon charging for 20s. Also, it is shown that the operation voltage and capacitance of flexible all-solid-state symmetric supercapacitors fabricated using aloe-derived activated carbon could be easily tuned by series and parallel combinations. The performance of fabricated supercapacitor devices using A. vera derived activated carbon in all-solid-state and ionic liquid indicates their viable applications in flexible devices and energy storage.

An electrochemical method for the determination of trace levels of mercury (II) ions using hydroxyapatite (HA) nanoparticles modified glassy carbon electrode (GCE) by square wave voltammetry is described for the first time. HA nanoparticles biosynthesized using Aloe vera plant (Av) extract exhibited improved electrocatalytic activity towards Hg 2+ ions when compared to pristine HA. The Av-HA modified GCE showed an excellent selectivity and improved sensitivity towards the detection of Hg 2+ without requiring preconcentration of mercury during voltammetric detection. Operational parameters such as pH, supporting electrolyte and scanning potential range were optimized. Under the optimum experimental conditions, the anodic peak current is proportional to the concentrations of mercury over a wide range of 2.0 × 10 -7 to 2.1 ×10 -4 M with the lowest detectable concentration of 141 nM.The Av-HA modified GCE showed good selectivity towards mercury in the presence of potential interferents such as copper, lead, cadmium, silver and zinc. The fabricated sensor displayed good reproducibility and is suitable for the determination of mercury in tap water and industry waste water.