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Electrochemical Supercapacitors for Energy Storage and Delivery
At the same time, a fundamental understanding of supercapacitor design,
operation, performance, and component optimization led to improvements
of supercapacitor performance, particularly increasing their energy density.
To further increase energy density, more advanced supercapacitors called
pseudocapacitors, in which the electroactive materials are composited with
carbon particles to form composite electrode materials, were developed.
The electrochemical reaction of the electroactive material in a pseudocapacitor takes place at the interface between the electrode and electrolyte via
adsorption, intercalation, or reduction–oxidation (redox) mechanisms. In
this way, the capacitance of the electrode and the energy density can be
increased significantly.
This chapter will provide a comprehensive discussion of the fundamentals
of double-layer supercapacitors, including the electric double-layer charging
and discharging mechanism, the theoretical principles that govern their
operation, and their structural designs.
2.2 Electrode and Electrolyte Interfaces and Their Capacitances
An electrochemical device consists of two electrodes with an electrolyte
between them. The electrolyte can be a solid or a solution. Solid state electrolytes serve two functions. They conduct ions and separate the positive
electrode from the negative electrode. For liquid state electrolytes such as
electrolyte solutions, an inert porous separator sheet allows the ions to pass
through, creating a conducting current. The structure of an electrochemical
capacitor is very similar to that of an electrochemical cell but there is no electron transfer across the interface.
Figure 2.1 shows a typical double-layer capacitor. On the positive electrode,
an accumulation of positive charges attracts an equal number of negative
charges around the electrode in the electrolyte side due to Coulomb’s force.
However, due to heat fluctuation in the electrolyte, the charges carried by
the ions have a scattering distribution, leading to some net negative charges
in the electrolyte zone near the electrode. The charge balance between the
electrode and the electrolyte represents an electric double-layer.
To maintain the electric neutrality of the system, an equal number of negative charges accumulates at the negative electrode near which is an equal
number of net positive charges in the adjacent electrolyte, forming another
double-layer. Therefore, a complete double-layer capacitor has two electric
double-layers, one at the positive electrode–electrolyte interface and the
other at the negative electrode–electrolyte interface. These two double-layers
constitute the capacitor’s “heart” and determine its performance. The following sections will present a theoretical description of the electric double-layer
in terms of capacitance for charge storage.
Electrochemical Supercapacitors for Energy Storage and Delivery
At the same time, a fundamental understanding of supercapacitor design,
operation, performance, and component optimization led to improvements
of supercapacitor performance, particularly increasing their energy density.
To further increase energy density, more advanced supercapacitors called
pseudocapacitors, in which the electroactive materials are composited with
carbon particles to form composite electrode materials, were developed.
The electrochemical reaction of the electroactive material in a pseudocapacitor takes place at the interface between the electrode and electrolyte via
adsorption, intercalation, or reduction–oxidation (redox) mechanisms. In
this way, the capacitance of the electrode and the energy density can be
increased significantly.
This chapter will provide a comprehensive discussion of the fundamentals
of double-layer supercapacitors, including the electric double-layer charging
and discharging mechanism, the theoretical principles that govern their
operation, and their structural designs.
2.2 Electrode and Electrolyte Interfaces and Their Capacitances
An electrochemical device consists of two electrodes with an electrolyte
between them. The electrolyte can be a solid or a solution. Solid state electrolytes serve two functions. They conduct ions and separate the positive
electrode from the negative electrode. For liquid state electrolytes such as
electrolyte solutions, an inert porous separator sheet allows the ions to pass
through, creating a conducting current. The structure of an electrochemical
capacitor is very similar to that of an electrochemical cell but there is no electron transfer across the interface.
Figure 2.1 shows a typical double-layer capacitor. On the positive electrode,
an accumulation of positive charges attracts an equal number of negative
charges around the electrode in the electrolyte side due to Coulomb’s force.
However, due to heat fluctuation in the electrolyte, the charges carried by
the ions have a scattering distribution, leading to some net negative charges
in the electrolyte zone near the electrode. The charge balance between the
electrode and the electrolyte represents an electric double-layer.
To maintain the electric neutrality of the system, an equal number of negative charges accumulates at the negative electrode near which is an equal
number of net positive charges in the adjacent electrolyte, forming another
double-layer. Therefore, a complete double-layer capacitor has two electric
double-layers, one at the positive electrode–electrolyte interface and the
other at the negative electrode–electrolyte interface. These two double-layers
constitute the capacitor’s “heart” and determine its performance. The following sections will present a theoretical description of the electric double-layer
in terms of capacitance for charge storage.