Hierarchical Nanostructures: Application to Supercapacitors - Hierarchical Nanostructures for Energy Devices - page 221

nanotubes to the underlying graphene. 70,71 Related to supercapacitors, only
a few exceptions of self-assembly approach for producing hierarchical
nanostructure have been reported. 72
The bottom-up synthesis processes consist of separated growth steps for
each hierarchical level. The growth of the first step mainly produces a
nanomaterial template that has larger porosity and better electrical con-
ductance. In the following steps, nanostructures of a smaller feature with
high SSA are produced on the template under equivalent or milder con-
ditions so as not to degrade the structures grown in the previous stages. In
the case of a unary system, the same precursor can be used, but in general,
the growth parameters are modified in the follow-up stages to induce dif-
ferent morphologies. In the case of a hybrid system, growth precursors and
conditions can be dramatically changed.
It has been reported that hierarchical nanostructures can overcome the
restrictions of nanomaterials produced by single-mode growth and achieve
unusual physical properties. Indeed, hierarchical nanostructures can im-
prove the performance of supercapacitors (i.e. higher energy density, higher
power density). A straightforward example is an increase of porosity or SSA
by addition of branch materials that expose a surface area larger than the
occupied sites of the mother material grown in the previous step.
As mentioned before, however, a one-dimensional approach with the aim
of increasing SSA does not necessarily lead to the performance improvement
of supercapacitors. The porosity of electrode materials can be classified
into the following categories based on the diameter of pores: micropores
( o 2 nm), mesopores (2-50 nm), and macropores (450 nm), according to
IUPAC. Fuertes et al. 73 suggested that micro- and meso-pores contribute to
the performance of supercapacitors in different manners: micropores are
mainly responsible for capacitance, whereas small mesopores (2-4 nm) fa-
cilitate propagation of charges for high current loads. Larger pores such as
macropores can act as accessible reservoirs of electrolyte ions. In this re-
spect, the pore structure of the electrode should be tightly controlled to
achieve optimal performance.
Hierarchical nanostructures potentially provide a larger degree of freedom
for structure control by means of the multi-stage synthesis processes. Due to
the separated growth for each hierarchical level, the porosity of hierarchical
nanomaterials can be more actively tailored. In general, the nanomaterial in
the first hierarchical level contains meso- or macro-pores, and its relatively
large size enables relatively fast electrical conductance. For instance,
mesoporous carbon nanomaterials, such as vertically aligned carbon nano-
tube arrays, can be a good candidate for the first level material. Nano-
structures of the next hierarchy have finer pores and play a central role in the
surface phenomena related to the charge storage. As a result, the resulting
porosity can follow a multi-modal distribution.
In addition to the tunable porosity, hybrid hierarchical nanostructures
consisting of different kinds of capacitive nanomaterials can benefit from
specialized functionalities of the constituent nanomaterials. Representative

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