Radial-Hierarchy Mesoporous Carbon Sphere with a Hollow Structure for High-Performance Supercapacitors

Supercapacitors are one of the most ideal energy storage systems due to their fast charge-discharge rate, stable cycle performance, and power density. As the electrode materials, the physicochemical properties including surface area, pore size, and structural morphology influence the electrochemical performance of the system. Therefore, significant improvement has been made in the research field of electrode materials for supercapacitors. Various materials could be considered as the proper electrode materials for supercapacitors; however, the carbonaceous materials are the most commonly used as electrodes owing to their large surface area, adjustable pore structure, and high electrical conductivity abundant availability, and excellent electric double-layer properties.<br/>To synthesize suitable carbonaceous materials for electric double-layer capacitors (EDLCs), the morphology of the porous structure is the main parameter. The carbonaceous materials which have proper pore size and structure provide a large surface area for accumulating electrostatic charges and a favorable diffusion pathway for electrolyte ions. Many previous studies have proven that mesoporous carbonaceous material can enhance their electrochemical properties such as adsorption, separation, catalysis, and energy storage capacity, thus broadening their applications. Especially ordered mesoporous carbons (OMCs) such as CMK, FDU, KIT, and MSU have received considerable attention in recent decades. However, the monotonous and relatively long porous channels diminish ion mobility in their form and result in the overall degradation of the energy storage system performance at the high current density.<br/>Herein, we report a facile method for the synthesis of a carbon allotrope based on a hollow sphere with a radial mesoporous hierarchy. There has been substantial research on both OMCs and hollow carbon spheres (HCSs), however, the combined structure of the thick mesoporous-shell and hollow-core has rarely been studied until recently. The mesoporous carbon hollow spheres (MCHSs) endow an ordered radial channel and space-saving stacking based on their spherical morphology. Compared to other carbonaceous materials, the MCHSs have the combined advantages of OMCs and shell-like carbon spheres, especially for application in supercapacitors.<br/>To synthesize the MCHSs, hierarchically and radially mesoporous silica was utilized as a template. The dandelion-shaped mesoporous silica spheres have been exploited in various fields since their first report. Here, the mesoporous edge of the dandelion-like silicas (DSSs) was directly replicated, whereas the core of the DSSs was engraved as a hollow structure. This is referred to as the “Dual-templating method.” To optimize the structure of the MCHSs, the composition of templates and precursors was changed. The optimal condition (MCHS-0.5) shows well-developed hierarchical and radial mesopores in the carbon shell and hollow core in the center of the carbon sphere. The carbon shell with narrow pore-size distribution and large surface area (1319 m²/g) provides a short ion-diffusion path and big specific capacitance. Also, the hollow structure endows the space for electrolyte penetration and retention. These structures play a role in improving their electrochemical performance in comparison to the existing carbonaceous materials. The MCHSs show a specific capacitance up to 135.9 F/g at 10 mV/s and present a well-developed EDLC shape at all scan rates (10 to 1000 mV/s) that qualified them as ideal electrode materials for EDLCs. The short pore length and radially interconnected pore structure combined with controlled surface chemistry would be beneficial for improving the energy density and electrical performance of supercapacitors without aggravating their high-power density and long lifecycle.