Environmental Engineering Reference
In-Depth Information
1.1.4 SUPER-CAPACITORS (EDLCS) AS NEWEST APPLICATION OF
CONTROLLING OF PORE SIZE IN POROUS CARBONS
The main objective of this paper is to provide a brief review of the pore size
control that is an important factor influencing application of activated carbon or
carbon nano structures adsorption in drinking water treatment or other adsorbent
application. Different pore sizes in carbon materials are required for their appli-
cations. Therefore, the PSD in carbon materials has to be controlled during their
preparation, by selecting the precursor, process and condition of carbonization,
and also those of activation. A wide range broad distribution in pore size and
shape is usually obtained in carbon materials. The control of pore size in carbons
is essential in order to compete in adsorption performance with porous inorganic
materials such as silica gels and zeolites, and to use the advantages of carbon
materials such as high chemical stability, high temperature resistance and low
weight. For applications in modern technology fields, not only high surface area
and large pore volume but also a sharp pore size distribution at a definite size and
control of surface nature of pore walls are strongly required. In order to control
the pore structure in carbon materials, studies on the selection of precursors and
preparation conditions have been extensively carried out and certain successes
have been achieved [1-3]. Pore sizes and their distributions in adsorbents have
to comply with requirements from different applications. Thus, relatively small
pores are needed for gas adsorption and relatively large pores for liquid adsorp-
tion, and a very narrow PSD is required for molecular sieving applications. Mac-
ropores in carbon materials were found to be effective for sorption of viscous
heavy oils. Recent novel techniques to control pore structure in carbon materials
can be expected to contribute to overcome this limitation [41, 46].
One of newest application that shows importance of pore structure control in
carbon materials, is an electric double-layer capacitor (EDLC) or super capacitor
that is an energy storage device that uses the electric double layer formed at the
interface between an electrode and the electrolyte. EDLCs are well documented
to exhibit significantly higher specific powers and longer cycle lifetimes com-
pared with those of most of rechargeable batteries, including lead acid, Ni-MH,
and Li-ion batteries. Hence, EDLCs have attracted considerable interest, given
the ever-increasing demands of electric vehicles, portable electronic devices, and
power sources for memory backup. The capacitance of an EDLC depends on the
surface area of the electrode materials. Therefore, activated carbons are neces-
sary materials for EDLC electrodes because of their large surface area, highly
porous structure, good adsorption properties, and high electrical conductivity. The
electrochemical performance of EDLCs is related to the surface area, the pore
structure, and the surface chemistry of the porous carbon. Various types of porous
carbon have been widely studied for use as electrode materials for EDLCs. Their
unusual structural and electronic properties make the carbon nanostructures ap-
plicable in, inter alia, the electrode materials of EDLCs and batteries. Activated
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