Hierarchical Nanostructures: Application to Supercapacitors - Hierarchical Nanostructures for Energy Devices - page 205

also spurring development of energy storage systems that can accommodate
high energy density in a compact device.
Electricity is the most valuable form of energy. The electricity can be
converted in a well-regulated manner to different forms of energy such
as thermal, radiant, chemical, and mechanical energies. As the promise of
zero-emission vehicles presents, utilization of the electricity can be free of
emissive pollutants. Furthermore, electrical energy powers popular elec-
tronics that are indispensable in our daily life.
The development of high performance electrical energy storage systems
has been a major concern in industry as well as in research societies.
Recently, modern mobile electronics are rapidly extending their capabilities
in order to accommodate large-data transfer and processing. Indeed, state-
of-the-art smart phones are small energy-intensive units with multicore
processors and high-resolution displays, and the performance requirement
continues to grow. Wearable computers are promising futuristic appli-
cations and are expected to require additional functionalities amenable to
mechanical deformation of the energy storage system.
Related to the development of advanced energy storage systems, two types
of energy storage systems have attracted particular interest: electrochemical
capacitors (so-called supercapacitors) and Li-ion batteries. The performance
of these devices has been evaluated routinely based on specific power vs.
specific energy relation and mapped in the so-called Ragone plot
(Figure 9.1). As clearly indicated in Figure 9.1, batteries have extended
discharge time with larger energy capacity. Thus batteries have been
dominantly used to power mobile electronics such as cell phones and laptops.
In comparison with the batteries, supercapacitors feature higher specific
power but relatively lower specific energy. The performance of super-
capacitors culminates in various situations where a burst of high power
output is required or relatively higher power should be supplied within a
short period of time (less than a minute). Likewise, supercapacitors can be
charged up in a comparably short time. Supercapacitors can also allow a very
large number of charge-discharge cycles (4500 000) with extended lifetime
compared with batteries and other conventional dielectric capacitors. 1,2
Owing to these characteristics, supercapacitors have been applied to
backup power devices, heavy hybrid electric vehicles for cyclic energy
capture, electric screwdrivers, and so on. 3
In this chapter, we focus on the working principles of supercapacitors and
application of nanomaterials, in particular those in the form of hierarchical
nanostructures. As will be discussed in the following sections, the energy
storage mechanism of supercapacitors is mostly based on surface
phenomena. In this regard, using nanomaterials that are distinguished by
extremely high surface to volume ratios offers a fundamental ground for
realizing unusually high capacitance energy storage systems. Moreover,
supercapacitor devices composed of nanomaterials can be light in weight,
optically transparent, 4 and flexible, 5 which are promising characteristics for
futuristic portable and wearable device applications. In the next section, we

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