Novel mesoporous carbon based nanostructures for energy storage applications

Abstract

Electrochemical energy storage systems (ESS), such as supercapacitors, batteries and fuel cells, can store and release electricity very efficiently, showing great promise as future energy storage systems. Electrode materials is the key elements of electrochemical ESS. Porous carbon materials, due to its high surface area, large pore volume and unique pore size distribution, have been viewed as good candidate for electrode materials of electrochemical ESS. Currently, activated carbon (AC) with a broad pore size distribution is commercialized as electrode materials for electrochemical application. However, the low conductivity, low mass transportation and low structural stability of AC have hindered its application in advanced electrochemical ESS. To this end, developing novel carbon nanostructures with tunable pore size, high conductivity and controlled morphology is critical for weaselling out of such predicament.The aims of this project includes the design and synthesis of novel mesoporous carbon materials for aqueous electrolyte, ionic liquid electrolyte based supercapacitor, and Li-S applications. In the respect of supercapacitor application, nitrogen-doped mesoporous carbon nanospheres and 2D mesoporous carbon nanosheets will be fabricated and their electrochemical properties will be investigated. In the respect of Li-S application, an activated microporous carbon encapsulated radial pore carbon sphere was rationally designed and its electrochemical properties will be investigated and compared with its counterparts.In the first part, we report the synthesis of N-doped MHCS using 3-aminophenol/ formaldehyde resin (APF) with a surfactant-free in situ silica-templating approach. The polymerization kinetics of the APF and tetraethoxysilane (TEOS) in the reaction system, as well as their cooperative assembly, can be adjusted by simply tuning the added amount of ethylenediamine. Because of the highly porous and thin shell, the final products displayed a unique invaginated sphere morphology, named as N-doped mesoporous invaginated carbon nanospheres (N-MICS) hereafter. As a demonstration of the application potential, N-MICS were applied as electrode materials for supercapacitors and exhibited excellent electrochemical performances with a high specific capacitance (261 F g-1), a superior cycling stability, and a good rate performance.In the second part, in order to set an idea platform to study the ion diffusivity and energy/power performances as a function of mesopore size, single-layered mesoporous carbon sandwiched graphene nanosheets (sMC@G) is synthesized for high-performance ionic liquid based supercapacitors. A new understanding of ion diffusion coefficient as a function of mesopore size is revealed. When using EMImBF4 as the electrolyte (cation size of 0.76 nm), the ion diffusion coefficient changes dramatically in the mesopore size range of 6.8-10.6 nm, then slightly increases when the pore size is further enlarged and approaches the bulk diffusion coefficient of the electrolyte. The power density of sMC@G with a pore size 10.6 nm is improved (14.7 kW kg-1 at 20 A g-1) without compromising the energy density (130 Wh kg-1 at 1 A g-1), superior to other porous carbon electrode materials in ionic liquid electrolyte supercapacitors.In the third part, nitrogen-doped hollow mesoporous carbon spheres (NHMCS) with tunable void space have been synthesized through ethylenediamine-assisted self-assembly approach between 3-aminophenol/formaldehyde resin (APF) and silica templates. The NHMCS with high specific surface area, large pore volume, interconnected radial mesopores and tunable void space. When used as the host material, the high specific surface area and large pore volume ensure high S loading; the interconnected radial pore provides low resistance and short diffusion pathway, facilitating fast mass transport to enhance the rate capability of the composite cathode; the tunable void space adjusts the volumetric energy density and offering enough free space for S expansion. Benefitting from such novel architecture of NHMCS, the obtained NHMCS/S cathode possess high S loading, improved electrochemical performance including high capacity, good rate performance and cycling stability.