In this study, using coconut fibers as raw material, activated carbon fibers were prepared via carbonization and KOH activation processes. The morphology, composition, specific surface area, pore structure and thermal stability of the resulting activated carbon fibers were systematically characterized. It was found that the activation process increases the specific surface area of carbon fibers to a greater extent via formation of a large number of micropores (0.7–1.8 nm) and a certain amount of slit-shaped mesopores (2–9 nm). The specific surface area and the pore volume of the activated carbon fibers reach 1556 m² g⁻¹ and 0.72 cm³ g⁻¹, respectively. The activation process can also decompose the tar deposits formed after the carbonization process by pyrolysis, making the surface of the activated carbon fibers smoother. To study the adsorption properties of the as-prepared activated carbon fibers, the adsorption capacities and adsorption kinetics of various organic dyes including methylene blue, Congo red and neutral red were investigated. The adsorption capacities of the dyes increased with the increasing initial dye concentrations, and varied greatly with the pH value of the system. In methylene blue and neutral red systems, the adsorption capacities reach the maximum at pH 9, and in the Congo red system, it reaches the maximum at pH 3. The adsorption capacities of the activated carbon fibers in methylene blue, Congo red and neutral red systems reached equilibrium at 150, 120, and 120 min, and the maximum adsorption capacities were 21.3, 22.1, and 20.7 mg g⁻¹, respectively. The kinetics of the adsorption process was investigated using three models including pseudo-first-order, pseudo-second-order and intraparticle diffusion models. The results indicated that the dynamic adsorption processes of coconut-based activated carbon fibers to methylene blue, Congo red and neutral red were all in accordance with the second-order kinetic model, and the equations are as follows: t/Q_t = 0.1028 + t/21.3220, t/Q_t = 0.1128 + t/21.5982 and t/Q_t = 0.0210 + t/20.6612.

A novel non-aqueous hybrid supercapacitor was fabricated from two spherical materials – an activated mesocarbon microbead (AMCMB) and MnO₂ nanowire-sphere, as the negative and positive electrodes, respectively. The preliminary results for this energy-storage device, which operates over a wide voltage range (0.0–3.0 V) using 1 M Et₄NBF₄ in acetonitrile (AN) as electrolytes, are presented. On the basis of a single electrode, the AMCMB|MnO₂ supercapacitor displays a high specific capacitance of 228 F g⁻¹ at a scan rate of 10 mV s⁻¹, corresponding to specific energy of 128 W h kg⁻¹ based on based on the total mass of active materials, while maintaining desirable cycling stability and rate capability. The combination of the spherical AMCMB and MnO₂ in a non-aqueous electrolyte is proved to be suitable for high-performance hybrid supercapacitor applications.

Hollow carbon nanocages (HCNCs) consisting of sp² carbon shells featured by a hollow interior cavity with defective microchannels (or customized mesopores) across the carbon shells, high specific surface area, and tunable electronic structure, are quilt different from the other nanocarbons such as carbon nanotubes and graphene. These structural and morphological characteristics make HCNCs a new platform for advanced electrochemical energy storage and conversion. This review focuses on the controllable preparation, structural regulation, and modification of HCNCs, as well as their electrochemical functions and applications as energy storage materials and electrocatalytic conversion materials. The metal single atoms‐functionalized structures and electrochemical properties of HCNCs are summarized systematically and deeply. The research challenges and trends are also envisaged for deepening and extending the study and application of this hollow carbon material. The development of multifunctional carbon‐based composite nanocages provides a new idea and method for improving the energy density, power density, and volume performance of electrochemical energy storage and conversion devices.

A new type of porous nano-MnO₂ for supercapacitors has been synthesized for the first time by a facile sonochemistry route from a quick-redox reaction between KMnO₄ and D-glucose. The crystal structure, morphology and chemical composition of the MnO₂ product were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS). The results show that the porous MnO₂ nanoparticles in the range 20–50 nm are interestingly composed of nanorods with diameters of about 2 nm and lengths of 4–8 nm. A possible growth mechanism of this nanostructure has been identified based on the experimental results. The electrochemical properties of the porous MnO₂ were investigated in a symmetric capacitor cell using both 1 M Na₂SO₄ and 9 M KOH aqueous solutions. The results indicate that the MnO₂ electrodes have a good capacitive performance in both cases.