Page 259 - Carbon Nanotube Fibres and Yarns

250 Carbon Nanotube Fibers and Yarns

For 1D (threadlike) and 2D (film or fabric) supercapacitors, it is some-
times more convenient to use areal and length specific capacitance, energy
and power than gravimetric performance indicators.

10.2 Electrochemical properties of CNTs

CNT can be considered as rolled graphene sheets with carbon atoms that are
2
covalently bonded with each other through sp hybridization. Depending on
the number of layers of graphene sheets, they are categorized as single-walled
CNTs (SWNTs) and multi-walled CNTs (MWNTs). CNTs have been
used as electrode materials in conventional SCs due to their unique struc-
2
ture, high surface area (typically over 1500 m /g), low mass density, outstand-
ing chemical stability, and excellent electronic conductivity [52–55]. They
are also widely used as electrode materials in flexible SCs with both liquid
electrolyte and polymer gel electrolyte. There are a number of advantages of
CNTs in SCs. CNTs are more efficient to percolate active particles than tra-
ditional carbon materials; they are often made into a porous network which
allows the ions to diffuse easily to the surface of the active moiety; and they
can help to alleviate the volumetric change during the charge and discharge,
resulting in improved cyclic performance. For bare CNTs, the reported spe-
2
cific surface areas are from 120 to 500 m /g, and the capacitances are ranged
from 5 to 200 F/g. CNT electrodes show lower equivalent series resistance
(ESR) than activated carbon due to a more efficient diffusion within the po-
rous network for the electrolyte ions, so they display acceptable capacitance
performance even at extremely high charging rates. For example, an aligned
MWNT sheet with Al sheet as current collector produced a discharge ca-
pacity of 10–15 F/g at an extremely high current density of 200 A/g, whereas
no discharge capacitance was obtained for commonly used activated carbon
electrode under the same condition [56]. For SWNTs, a specific capacitance
−1
of 180 F/g and a power density of 20 kW kg were obtained at an energy
−1
density of 7 Wh kg [57].
CNTs can be brush−/spray-coated directly onto either flexible
nano-conductive substrates as both electrode and current collector [58–61],
or used as flexible conductive substrates for electrodes [62]. In this regard,
Kaempgen and coworkers [58] reported printable thin film SC using spray-
coated SWNTs on polyethylene-terephthalate (PET) films as both elec-
trodes and charge collectors. In order to make the device fully printable,
gel electrolyte (PVA/H 3 PO 4 ) was used to combine the separator and the
electrolyte into a single layer. The CNT electrodes and the gel electrolyte