High-quality ultrathin two-dimensional nanosheets of α-Ni(OH)₂ are synthesized at large scale via microwave-assisted liquid-phase growth under low-temperature atmospheric conditions. After heat treatment, non-layered NiO nanosheets are obtained while maintaining their original frame structure. The well-defined and freestanding nanosheets exhibit a micron-sized planar area and ultrathin thickness (<2 nm), suggesting an ultrahigh surface atom ratio with unique surface and electronic structure. The ultrathin 2D nanostructure can make most atoms exposed outside with high activity thus facilitate the surface-dependent electrochemical reaction processes. The ultrathin α-Ni(OH)₂ and NiO nanosheets exhibit enhanced supercapacitor performances. Particularly, the α-Ni(OH)₂ nanosheets exhibit a maximum specific capacitance of 4172.5 F g⁻¹ at a current density of 1 A g⁻¹. Even at higher rate of 16 A g⁻¹, the specific capacitance is still maintained at 2680 F g⁻¹ with 98.5% retention after 2000 cycles. Even more important, we develop a facile and scalable method to produce high-quality ultrathin transition metal hydroxide and oxide nanosheets and make a possibility in commercial applications.

We have established a facile and scaleable approach to fabricate tubular graphitic-C₃N₄ using melamine. The construction of the unique tubular morphology is a result of the pre-treatment of melamine with HNO₃. Herein, for the first time, we have explored the electrochemical properties of g-C₃N₄ as an electrode material for supercapacitors. Tubular g-C₃N₄ has significant advantages due to its distinctive morphology, high surface area (182.61 m² g⁻¹) and combination of carbon with nitrogen. Therefore, tubular g-C₃N₄ demonstrated a good specific capacitance of 233 F g⁻¹ at a current density of 0.2 A g⁻¹ in 6 M KOH electrolyte. Furthermore, tubular g-C₃N₄ maintained a high capacitance retention capability (90%) after 1000 cycles. The photocatalytic activity of tubular g-C₃N₄ was evaluated using the organic dyes such as Methylene Blue (MB) and Methylene Orange (MO) under visible light. Tubular g-C₃N₄ demonstrated good photocatalytic activity and enhanced stability compared to bulk g-C₃N₄. The enhanced performance is because of the high surface area, which contains more active sites for reaction. The encouraging performance of tubular g-C₃N₄ in supercapacitors and as a photocatalyst points toward it being a prospective material for energy storage that is environmentally clean.

Large areal capacitance is essentially required to integrate the energy storage devices at the microscale electronic appliances. Energy storage devices based on metal oxides are mostly fabricated with low mass loading per unit area which demonstrated low areal capacitance. It is still a challenge to fabricate supercapacitor devices of porous metal oxides with large areal capacitance. Herein we report microwave method followed by a pyrolysis of the as-prepared precursor is used to synthesize porous nickel cobaltite microspheres. Porous NiCo₂O₄ microspheres are capable to deliver large areal capacitance due to their high specific surface area and small crystallite size. The facile strategy is successfully demonstrated to fabricate aqueous-based asymmetric & symmetric supercapacitor devices of porous NiCo₂O₄ microspheres with high mass loading of electroactive materials. The asymmetric & symmetric devices exhibit maximum areal capacitance and energy density of 380 mF cm⁻² & 19.1 Wh Kg⁻¹ and 194 mF cm⁻² & 4.5 Wh Kg⁻¹ (based on total mass loading of 6.25 & 6.0 mg) respectively at current density of 1 mA cm⁻². The successful fabrication of symmetric device also indicates that NiCo₂O₄ can also be used as the negative electrode material for futuristic asymmetric devices.