Environmental Engineering Reference
In-Depth Information
relationship are:(i) assumptions in the measurement of electrode surface-area; (ii)
variations in the specific capacitance of carbons with differing morphology; (iii)
variations in surface chemistry (wettability and pseudo capacitive contributions),
(iv) variations in the conditions under which carbon capacitance is measured. The
surface areas of porous carbons and electrodes are most commonly measured by
gas adsorption (usually nitrogen at 77 K) and use BET theory to convert adsorp-
tion data into an estimate of apparent surface area. Despite its widespread use,
the application of this approach to highly porous (particularly microporous) and
heterogeneous materials has some limitations and is perhaps more appropriately
used as a semi-quantitative tool. Possibly the greatest constraint in attempting to
correlate capacitance with BET surface area, is the assumption that the surface
area accessed by nitrogen gas is similar to the surface accessed by the electrolyte
during the measurement of capacitance. While gas adsorption can be expected to
penetrate the majority of open pores down to a size that approaches the molecular
size of the adsorbate, electrolyte accessibility will be more sensitive to variations
in carbon structure and surface properties. Electrolyte penetration into fine pores,
particularly by larger organic electrolytes, is expected to be more restricted (due
to ion sieving effects) and vary considerably with the electrolyte used. Variations
in electrolyte-electrode surface interactions that arise from differing electrolyte
properties (viscosity, dielectric constant, dipole moment) will also influence wet-
tability and, hence, electrolyte penetration into pores [64, 68].
1.1.11.4.2
SUPER-CAPACITORS BASED ON CNTS
The presence of mesopores in electrodes based on CNTs, due to the central canal
and entanglement enables easy access of ions from electrolyte. For electrodes
built from multi-walled carbon nanotubes (MWCNTs), specific capacitance in a
range of 4-135 F/g was found in Refs. For single-walled carbon nanotubes (SW-
CNTs) a maximum specific capacitance of 180 F/g and a measured power density
of 20 kW/kg at the energy density of 7Wh/ kg, in KOH electrolyte is reported. In
other work an initial specific capacitance of 128 F/g decreased after charging-dis-
charging cycles to 58 F/g. Enhancement of specific capacitance given by CNTs is
possible For example, by their mixing with conducting polypyr role. A compara-
tive investigation of the specific capacitance achieved with CNTs and activated
carbon material reveals the fact activated carbon material exhibited significantly
higher capacitance. SWCNTs/polypyr role nanocomposite electrode used in re-
cent work indicated much higher specific capacitance than pure polypyr role or
SWCNTs electrodes. Super capacitor electrodes based on carbon nanotube-poly-
aniline (CNT-PANI) nano composite by coating polyaniline on the surface of the
CNT have been used recently. At a current density of 10 mA/cm, the CNT-PANI
nano composite exhibits high specific capacitance of 201 F/g, in comparison with
a value of 52 F/g for the CNT. The super capacitors based on the CNT-PANI
Search WWH ::
Custom Search