Carbon Nanotubes Carbon Nanotubes (CNTS) were first discovered by Iijima
at the NEC laboratory in 1991 [2]. Fig. 2 (left) shows high resolution TEM
micrographs of the concentric wall structure of the multi wall nanotubes
(MWNTs) first observed by Iijima [2]. Single wall nanotubes (SWNTs) are
composed of a single rolled up graphene layer to form a cylindrical wall one
carbon atom thick.
Fig. 2:
(Left) TEM Micrographs of multi wall coaxial nanotubes with various inner and
outer diameters, di and do, and numbers of cylindrical shells N reported by Ilijima in 1991:
(A) N = 5, do = 6.7nm; (B) N = 2, do = 5.5nm; and (C) N = 7, di = 2.3nm, do = 6.5nm.
(Right) SEM micrograph of a film of vertically aligned CNTs with estimated density of
107tubes/mm2.
Ballistic Conduction in Carbon Nanotubes The conduction phenomena in
SWNTs and MWNTs with diameters smaller than 50nm is ballistic over lengths
of several microns. Although for CNTs longer the few tens of microns the
conduction is diffusive, the conductivity in CNTs hundreds of microns long is
still more than two orders of magnitude higher than silicon. In an ultracapacitor
this leads to reduced internal resistance and high power density.
Fig. 3: (A) SEM micrograph of the top surface of an activated carbon based electrode.
(B) SEM micrograph of the cross section of an activated carbon based electrode.
Nanotube Enhanced Ultracapacitor A matrix of vertically aligned carbon
nanotube (CNT) has been investigated as a DLC electrode (see Fig. 2 right).
Our analysis shows that this configuration can provide a combination of high
power density (more than four orders of magnitude greater than fuel cells) and
energy density (comparable to Li-Ion batteries). The significant enhancement
in the achievable DLC power density derives from the high conductivity
obtainable with CNTs, which in the limit of a few microns in length present
ballistic conduction. The energy density improvement of a “nanotube enhanced
electrode” is due to the higher effective surface area obtain