Hierarchical Nanostructures: Application to Supercapacitors - Hierarchical Nanostructures for Energy Devices - page 220

Nevertheless, the conduction properties and the mechanical strength do
not match those of carbon-based nanomaterials. As discussed in section
9.3.3, the electroactive nanomaterials can be loaded on foreign materials
that can compensate for the listed problems. The concept of a hybrid elec-
trode system was born as a novel technical solution to combine the advan-
tages of EDLCs (high specific power) with pseudocapacitors (high specific
energy). In the next section, examples of the hybrid electrode are introduced
with an emphasis on hierarchical arrangement of electrode materials.

d n 3 r 4 n g | 4

9.3.5 Hierarchical Nanostructures for Supercapacitors
As consistently discussed in this topic, hierarchical nanostructures have
opened a new route for realizing advanced energy materials. The constituent
nanomaterials can be a single (unary system) or different (multi-component
system: binary, ternary, and more) material(s) with different morphological
attributes at each hierarchical level. Branched nanowires exemplify hier-
archical nanostructures. 68 An intriguing expression, nanoforest, could be an
analogical name of a hierarchical nanostructure. 69
Hierarchical structures for supercapacitors are synthesized mostly based
on the bottom-up approach because of fine control of the follow-up structure
and better electrical contact between different hierarchical levels. For in-
stance, a totally graphitic structure can be obtained by growing vertically
aligned carbon nanotube arrays on graphene (Figure 9.9). The seamless
contact at the interface leads to ecient electron collection from the

.

Figure 9.9 Illustration of growth processes in the catalytic CVD synthesis of a CNT-

graphene structure. (a) Ni catalyst-assisted hydrogenation of graphitic
carbon caused by hydrogen abundant gases (or absence of carbon
feedstock). (b) Seamless connection of CNTs and the underlying gra-
phene obtained by successful catalytic growth of CNTs in a methane
carbon feedstock gas. r IOP Publishing.
Reproduced from ref. 71 by permission of IOP Publishing. All rights
reserved.