3
Fundamentals of Electrochemical
Pseudocapacitors
3.1 Introduction
As discussed in the previous chapter, the double-layer capacitance developed
in electrochemical supercapacitors (ESs) is mainly due to net electrostatic
charge accumulation and separation at the electrode–electrolyte interface.
The net negative (or negative) charges such as electrons are accumulated
near the electrode surface. At the same time, an equal number of positive
charges such as cations are accumulated near the electrode surface at the
electrolyte side, forming electric double-layers such as the Helmholtz and
diffuse layers [1].
Both layers are responsible for and contribute to the magnitude of the
capacitance. Because the magnitude of the capacitance is strongly dependent
on the electrode surface, materials having large surface areas such as active
carbon are needed to construct a layer forming a porous electrode. However,
even with a porous electrode configuration, double-layer capacitance is relatively small because only the carbon particle surface can physically store
charges and the electrolyte ion accessibility of the porous structure is limited. Most carbon particles in those inaccessible areas are useless for charge
storage.
To increase the capacitance of ESs, some electrochemically active materials
are explored for electrode use to provide much higher pseudocapacitance
than double-layer capacitance. Pseudocapacitive charge storage fundamentally differs from the electrostatic mechanism that governs double-layer
capacitance. For pseudocapacitance, a faradic charge transfer in the electrode
porous layer occurs through a thermodynamically and kinetically favored
electrochemical reduction–oxidation (redox) reaction [1].
Because this redox reaction is strongly dependent on the electrode potential, the change in charge quantity arising from this reaction (dq) has a relationship with the change in electrode potential (dV). The dependency of dq
on dV (dq/dV) is called the pseudocapacitance created by the redox reaction.
99
Fundamentals of Electrochemical
Pseudocapacitors
3.1 Introduction
As discussed in the previous chapter, the double-layer capacitance developed
in electrochemical supercapacitors (ESs) is mainly due to net electrostatic
charge accumulation and separation at the electrode–electrolyte interface.
The net negative (or negative) charges such as electrons are accumulated
near the electrode surface. At the same time, an equal number of positive
charges such as cations are accumulated near the electrode surface at the
electrolyte side, forming electric double-layers such as the Helmholtz and
diffuse layers [1].
Both layers are responsible for and contribute to the magnitude of the
capacitance. Because the magnitude of the capacitance is strongly dependent
on the electrode surface, materials having large surface areas such as active
carbon are needed to construct a layer forming a porous electrode. However,
even with a porous electrode configuration, double-layer capacitance is relatively small because only the carbon particle surface can physically store
charges and the electrolyte ion accessibility of the porous structure is limited. Most carbon particles in those inaccessible areas are useless for charge
storage.
To increase the capacitance of ESs, some electrochemically active materials
are explored for electrode use to provide much higher pseudocapacitance
than double-layer capacitance. Pseudocapacitive charge storage fundamentally differs from the electrostatic mechanism that governs double-layer
capacitance. For pseudocapacitance, a faradic charge transfer in the electrode
porous layer occurs through a thermodynamically and kinetically favored
electrochemical reduction–oxidation (redox) reaction [1].
Because this redox reaction is strongly dependent on the electrode potential, the change in charge quantity arising from this reaction (dq) has a relationship with the change in electrode potential (dV). The dependency of dq
on dV (dq/dV) is called the pseudocapacitance created by the redox reaction.
99