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.

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.

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MXenes are the class of two-dimensional transition metal carbides and nitrides that exhibit unique properties and are used in a multitude of applications such as biosensors, water purification, electromagnetic interference shielding, electrocatalysis, supercapacitors, and so forth. Carbide-based MXenes are being widely explored, whereas investigations on nitride-based ones are seldom. Among the nitride-based MXenes obtained from their MAX phases, only Ti 4 N 3 and Ti 2 N are reported so far. Herein, we report a novel synthesis of V 2 NT x (T x is the surface termination) obtained by the selective removal of “Al” from V 2 AlN by immersing powders of V 2 AlN in the LiF–HCl mixture (salt–acid etching) followed by sonication to obtain V 2 NT x (T x = −F, −O) MXene which is then delaminated using the dimethyl sulfoxide solvent. The V 2 NT x MXene is characterized by X-ray diffraction studies, field emission scanning electron microscope imaging, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and high-resolution transmission electron microscope imaging. Supercapacitor electrodes are prepared using V 2 NT x MXenes and their electrochemical performances are examined by cyclic voltammetry, galvanostatic charge/discharge measurement, and electrochemical impedance spectroscopy. The V 2 NT x MXene electrode exhibits a specific capacitance of 112.8 F/g at a current density of 1.85 mA/cm 2 with an energy and power density of 15.66 W h/kg and 3748.4 W/kg, respectively, in 3.5 M KOH aqueous electrolyte. The electrode exhibits an excellent capacitance retention of 96% even after 10,000 charge/discharge cycles. An asymmetric supercapacitor fabricated with V 2 NT x as a negative electrode and Mn 3 O 4 nanowalls as a positive electrode helps obtain a cell voltage of 1.8 V in aqueous KOH electrolyte.

Owing to our inevitable energy necessity, alternate energy resources have to be implemented in our energy storage systems to enervate the energy demands. On the consideration of the above statement, activated high surface area three‐dimensional (3D) nanoporous carbon derived from the orange peel bio‐waste has been studied for symmetric flexible and bendable solid state supercapacitor (SSC) with high energy density. The nitrogen adsorption/desorption isotherms revealed that the activated nano‐porous carbon exhibits a high specific surface area and average pore volume of 2160 m2/g and 0.779 cc/g with the uniform meso and microporous network. The fabricated symmetric cell using the activated porous carbon in aqueous electrolyte exhibits a high specific capacitance of 460 F/g at 1 A/g with an excellent electrochemical stability of 98% for 10000 cycles and virtuous rate performance at higher current densities. The fabricated aqueous symmetric device exhibits a high energy density and power density of 12 Wh/kg and 32.8 kW/kg, respectively. Similarly, the symmetric device in ionic liquid electrolyte shows a high cell voltage of 3 V and high energy and power density of 43 Wh/kg and 1185 W/kg, respectively. Also, fabricated all‐solid‐state flexible supercapacitor device exhibits a high energy and power density of 11.4 Wh/kg and 6.6 kW/kg, respectively. The flexible supercapacitor device was shown to light up a red light emitting diode (LED) for more than three minutes.

The 2D/2D layered materials are gaining much‐needed attention owing to the unprecedented results in supercapacitors by their robust structural and electrochemical compatibility. Here, a facile scalable synthesis of 2D/2D MXene/boron carbon nitride (BCN) heterostructure through no residue direct pyrolysis is reported. The process allows the in‐situ growth of BCN nanosheets unravelling the surfaces of MXene synergistically that provide an interconnected conductive network with wide potential window, augmented proportion of Ti sites at elevated temperature removing terminal groups enabling high pseudocapacitive activity and impressive stability. As a result, the as‐assembled MXene/BCN electrode records a high specific capacitance of 1173 F g−1 (1876 C g−1) at 2 A g−1 and an energy density of 45 Wh kg−1. Further, the fabricated solid‐state device exhibits an ultra‐high cyclability of 100% capacitive retention after 100 000 cycles. This will be an epitome for future 2D/2D heterostructures with commendable electrochemical properties as an expedient solution for energy storage applications.

The effect of electrolytes on activated porous carbon was extensively studied using different electrolytes. A symmetric supercapacitor cell in redox additive electrolyte delivered a high energy (58.5 W h kg−1) and power density (9 kW kg−1).

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.