Hierarchical Nanostructures: Application to Supercapacitors - Hierarchical Nanostructures for Energy Devices - page 211

of ESR and hence decrease of specific power (eqn (9.5)). Therefore, a tech-
nical trade-off between porosity and conductance should be reasonably
made based on an understanding of the ion transport mechanism through
nanoscale pores.
The potential window is another important parameter that determines the
specific energy and power densities. As eqn (9.4) and (9.5) indicate, the
potential window has an influence on the performance by the second order.
Thus increasing the potential window is a very effective way to enhance
the energy/power density of EDLC. In the case of aqueous electrolytes, the
potential window is restricted below 1.23 V, above which electrolysis of water
molecules limits normal operation. In spite of the toxicity, organic electro-
lytes allow a larger potential (i.e. acetonitrile: 2.0 V). Recently, ionic liquid
electrolytes are being actively investigated, due to their even larger voltage
window (

d n 3 r 4 n g | 4

4.0 V).
There exists a new type of EDLC: a micro-supercapacitor. In contrast to the
conventional structure of EDLC, the electrodes are not facing each other, but
are placed in the same plane. To enhance the specific capacitance, micro-
supercapacitors adopt a characteristic interdigitated finger structure.
Figure 9.2(c) shows a typical electrode structure of micro-supercapacitors.
Calculating the capacitance of micro-supercapacitors is not as straight-
forward as that of traditional capacitors (eqn (9.1)). Geometric parameters of
the finger structure should be comprehensively considered in conjunction
with the conventional parameters. Readers are encouraged to refer to ref. 15
for a detailed discussion.
EDLC is distinguished from batteries in that no electron transfer occurs
across the electrode-electrolyte interface (non-faradaic). As discussed above,
the nature of energy storage is purely electrostatic. Thus, fast separation and
recombination of charge (high specific power) are enabled, whereas
charging-discharging rates of batteries are limited by relatively slow
electrochemical redox reaction kinetics. Electrodes of EDLCs are relatively
conductive and thereby generate minimal heat compared with batteries.
EDLCs are also free of electrode dissolution, so their lifetime can be
extended to a million cycles.
Notwithstanding the high power delivery and other merits, the perform-
ance of EDLC is still limited in the amount of stored energy (Figure 9.1).
Therefore, a new form of supercapacitors has been investigated: the so-
called pseudocapacitors. In the next section, the characteristics and energy
storage mechanism of pseudocapacitors are discussed.

B

.

9.2.2 Pseudocapacitor
Pseudocapacitors inherit many useful features of EDLCs. However, the
electrode of pseudocapacitors is not chemically inert. In pseudocapacitors,
reversible redox reactions with the electrolyte occur near the electrode sur-
face (faradaic). However, the reaction domain is not limited to the very
surface, proceeding into the bulk of the material. Thus, pseudocapacitors