Carbon Structures with Hollow Internal Cavity as Charge Storage Materials to Achieve High Energy Density

Abstract

Hollow porous carbon spheres (HPCS) have been synthesized by using spherical silica nanoparticles (S–SiO2) as templates. S–SiO2 nanoparticles have been coated first with the polymer of phloroglucinol/1,4-phenylenediamine/formaldehyde (PPF), followed by a second layer of SiO2, and again with PPF as the third layer. After each step of coating, the sample has been pyrolyzed under nitrogen and S–SiO2 has been removed to obtain HPCS-I, HPCS-II, and HPCS-III, from the first, second, and third coated samples, respectively. The synthetic strategy relies on the use of a hard template to create void spherical cores and space confinement to develop shells of the hollow spheres. All of the materials show a uniform spherical morphology with a hollow inner core generated by the removal of the SiO2 template. The samples have been characterized by thermal analysis, powder X-ray diffraction, nitrogen adsorption/desorption studies, electron microscopy, and X-ray photoelectron spectroscopy. Among the samples, the structural formation of HPCS-II is found to be superior and it is also manifested in its electrochemical properties. While all of the samples exhibit near-rectangular cyclic voltammograms, the specific capacitance of HPCS-II is found to be the highest. Galvanostatic charge/discharge (GCD) studies also support the observation, and the specific capacitance is found to be 592 F·g–1 at a current density of 1 A·g–1, which is retained at 444 F·g–1 even at a very high current density of 100 A·g–1. The major contribution toward such electrochemical behavior is believed to arise from electrical double-layer capacitance (EDLC). HPCS-II is found to be highly stable, retaining 100% of its capacitance value at least up to 5000 GCD cycles. The power density of HPCS-II is nearly thrice the power target value projected by the Partnership for a New Generation of Vehicles (PNGV), and it shows an outstanding energy density value.