Increasing energy density of electrochemical capacitors (ECs)

Increasing energy density of electrochemical capacitors (ECs) is crucial for their
applications in energy storage devices requiring short peak power pulses as well as longterm operation. ECs are operated via two primary charge mechanisms, that is, the
electrochemical double-layer capacitance and the pseudocapacitance. In the thesis, the
carbon materials and LiMn2O4, which generate double-layer capacitance and
pseudocapacitance, respectively, have been investigated. The effects of specific surface
area, pore structure and surface functionality on the energy storage performance of ECS
have been studied.
Micro-porous (<2 nm) carbon with pores inaccessible to the solvated ions may
limit the ion diffusion, resulting in a low rate capability. Hence this work attempts to
generate hierarchical macropores/mesopores/micropores in the electrode material.
Flexible, self-sustained and hierarchical porous carbon nanofibers (CNFs) are fabricated
using terephthalic acid as the sacrificial agent. After sublimation and carbonization, the
electrospun mat is converted to a hierarchical porous carbon framework. The high
specific capacitance and good rate capability are associated with the unique hierarchical
porous structure of the as-prepared CNFs. Both the outer fiber surface and inner porous
structure can be accessible for charge accumulation through pores on the surface.
Hierarchical macropores/mesopores in the fiber also help accelerate the ion-diffusion into
inner micropores.
Besides fossil resources, renewable biomass has also been explored as the source
material for supercapacitors in the present work. Lignin, the major aromatic constituent
of plant and woods, is utilized as the carbon precursor to prepare the mesoporous ligninchar. The lignin-derived carbon is prepared by taking an advantage of the organic-organic
self-assembly method, which allows the direct formation of mesoporous polymer
composite from carbon precursor and block copolymer, and conversion to porous carbon
by carbonization. Hierarchically porous carbon (HPC) with pores at different scales has
been obtained after alkali activation. The experimental results show that the appropriate
pore size distribution can ensure high power density and high energy density due to the
short diffusion distance and the minimized electric resistance. Utilization of biomass as
the source materials for supercapacitors will reduce the costs for fabrication of energy
storage devices.