Hierarchical Nanostructures: Application to Supercapacitors - Hierarchical Nanostructures for Energy Devices - page 210

using high SSA electrodes should be a straightforward approach to enhance
capacitance. As will be discussed in section 9.3, various carbon nanoma-
terials, such as carbon nanotubes and graphene, have very high SSA. In this
regard, porous assemblies of these carbon nanomaterials can be promising
electrodes for high capacitance EDLCs.
In addition, the pore size of the supercapacitor electrode should be
carefully tuned to achieve higher specific capacitance. While eqn (9.1)
accounts for the capacitance of conventional planar electrodes, it is not
directly applicable for calculation of the capacitance of nanoscale pores. 11 As
revealed by Chmiola et al., 12,13 when normalized by SSA, the capacitance of
nanoscale pores experiences an abrupt increase as the pore size decreases
below the size of solvated ions. They attributed this unexpected dramatic
increase to desolvation of the ions in the extremely confined pore space and
subsequent closer approach of the ions to the pore wall. As the pore size
decreases further and approaches the size of electrolyte ions, size exclusion
starts hindering the ions from accessing the surface inside the pore. As a
result, the capacitance diminishes quickly. A similar trend was reported
from a study on EDLC with an ionic liquid electrolyte, which is free of
solvation/desolvation effects (Figure 9.4). 14
It should also be considered that increasing specific capacitance can cause
degradation of power performance. For instance, increased porosity of EDLC
electrodes can compromise electrical conductance, resulting in an increase

d n 3 r 4 n g | 4

.

Figure 9.4 Capacitance change by variation of the pore size of carbide-derived
carbon. The capacitance was normalized by the SSA of the electrode.
An ethyl-methylimidazolium/bis(trifluoro-methane-sulfonyl)imide ionic
liquid (EMI/TFSI) electrolyte was applied to exclude effects of solvation/
desolvation of ions.
Reprinted with permission from ref. 14. Copyright 2008 American
Chemical Society.