Hierarchical Nanostructures: Application to Supercapacitors - Hierarchical Nanostructures for Energy Devices - page 213

d n 3 r 4 n g | 4

Figure 9.5 Chemical scheme of faradaic charge storage. (a) General conductive
polymer, (b,c) polyaniline (PANI) (0 r a r 1).
Reproduced from ref. 24.

The synthesis of conductive polymers is cheap and highly scalable. These
materials have similar electrochemical characteristics to pseudocapacitive
metal oxides. During charging-discharging, electrolyte ions move into or out
of the backbone of the polymer via reversible redox reactions (Figure 9.5). 24
As is the case with metal oxides, the reaction domain proceeds into the bulk
of the material as the electrode is charged. The intercalation and deinter-
calation of ions in the polymer electrode can cause cyclic swelling and
shrinking, resulting in mechanical failure of the electrode. This stability
issue limits the lifetimes of conductive polymer electrodes.
Pseudocapacitors resemble batteries in terms of their charge storage
mechanism, and their performance is between those of EDLCs and batteries.
The most valued feature of pseudocapacitors is their increased capacitance
and thereby the increased energy density compared with EDLC. As expected,
however, the relatively slow redox reaction kinetics and poor conductivity of
electroactive materials can undermine the power performance of pseudo-
capacitors. To achieve high-energy storage and high-power output simul-
taneously, advanced material engineering has pursued the development of
hybrid electrode systems. In the following sections, we will discuss various
nanomaterials for electrodes of supercapacitors and their integration into
novel hybrid electrode systems.

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9.3 Nanomaterials for Supercapacitor Electrodes
As mentioned, assembled porous nanomaterials have an extraordinarily
high specific surface area (SSA). They can be a very attractive choice for the
electrodes of supercapacitors. A large degree of freedom in the design of the
electrode and the selection of the material has spurred comprehensive re-
search and performance evaluation of a variety of nanostructured electrodes.
In the following sections, various nanomaterials especially used for elec-
trodes of supercapacitors are introduced and their technical importance is
discussed. Figure 9.6 shows a brief summary of the specific capacitance of
supercapacitor electrodes of various materials. It should be especially noted
that the performance of supercapacitors (capacitance, specific power,
specific energy) of various nanomaterials can be quite scattered, depending