Hierarchical Nanostructures: Application to Supercapacitors - Hierarchical Nanostructures for Energy Devices - page 207

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Figure 9.2 Structures of supercapacitors. (a) Conventional structure of electrochem-

ical double layer supercapacitors. (b-c) Typical electrode structure
of micro-supercapacitors. (b) Photo image of a real flexible micro-
supercapacitor. (c) Schematic of typical interdigitated electrodes of
micro-supercapacitors. (b-c) Reproduced from ref. 7.

.

EC consisting of these basic components. A differential potential externally
imposed on the electrodes induces an electric field across the electrolyte.
The electrodes provide electrical channels for electrons to be delivered to the
interface with the electrolyte. To minimize the overall resistance the elec-
trodes are connected to conductive (generally metallic) current collectors
that lead to the external terminals. Electrolytes contain ionic molecules in
liquid- or solid-state media. Separators are porous insulators that allow the
transport of ions but prevent electrical short. However, the separator is not
required with solid-state electrolytes since the electrolyte itself blocks
possible contact between the anode and cathode. The separator is also not
needed in micro-supercapacitors because of the separate patterns of inter-
digitated electrodes (Figure 9.2(b),(c)).
In EDLC, ionic molecules of the electrolyte are accumulated by coulombic
attraction at the electrode surface. At the very surface of the electrode, they
form a thin layer: the so-called Helmoltz layer. Along with this layer, an ion
distribution can be further developed toward the electrolyte side, due to
thermal fluctuations. This relatively thick layer is called the diffuse layer.
These two layers constitute the essence of the charge storage mechanism of
EDLC and play a major role in determining the electrochemical performance
of the capacitors.