Hierarchical Nanostructures: Application to Supercapacitors - Hierarchical Nanostructures for Energy Devices - page 219

d n 3 r 4 n g | 4

Figure 9.8 Interconnected graphitic carbon structure derived from hemp. (a) Sche-

matic of the fabrication process. (b) SEM of the interconnected porous
carbon structure. (c) TEM of the carbon structure. (d) High-resolution
TEM of the corresponding graphitic carbon. (e) Annular dark field (ADF)
TEM image and electron energy loss spectroscopy (EELS) thickness
profile (inset) of the carbon nanosheet.
Reprinted with permission from ref. 59. Copyright 2013 American
Chemical Society.

supercapacitor electrodes. Nanomaterials of precious metal elements could be
candidates. However, the very high price prevents the direct use of precious
metals as electrodes. Instead, precious metal nanomaterials can be used as an
additive to significantly enhance the conductivity of carbon-based electrodes. 64
In the composites of nanomaterials, multiple heterogeneous nanomater-
ials are dispersed uniformly or stacked layer by layer. In other cases, the
component nanomaterials can be assembled into an intriguing hierarchical
structure. Further details of the hybrid hierarchical nanostructure will be
discussed in section 9.3.5.

.

9.3.4 Electroactive Nanomaterials for Pseudocapacitors
Electroactive materials for pseudocapacitors (section 9.2.2) can be also
synthesized in the form of nanomaterials such as nanoparticles, 65 nano-
wires, 66 and nanosheets. 16 These structures can be fabricated by hydro-
thermal methods 67 or electrodeposition on conductive supports.
Consistently, supercapacitor electrodes of these materials benefit from
significantly increased porosity and SSA and high pseudocapacitance.
A decrease in the size can enhance the mechanical stability under cyclic
charge-discharge processes. 19 Nanostructures of electroactive materials can
also potentially overcome the intrinsic low conductivity owing to their short
conduction path. For instance, nanoparticles of electroactive metal oxides,
which feature high SSA, can provide a sucient electrical conductance when
supported on a conductive current collector.