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Electrochemical Supercapacitor Design, Fabrication, and Operation
power supplies or hybrid electric vehicles, contains numerous ~2.7 V single
cells in series.
In ES stack design and manufacturing, maximizing the safe operating
voltage of each cell is a priority, along with reducing costs by minimizing
material requirements, using cost-effective materials, and simplifying manufacturing processes. In general, a wide ES operating voltage improves the
efficiency of storing energy and supplying power. For example, electronic
systems generally possess a threshold for a minimum operating voltage that
directly relates to the usable power stored in an electrochemical capacitor
cell. At the current technological state, utilization of 75% of the stored energy
can be achieved when an ES is discharged from an initial charge potential
V initial to one half of V initial .
In ES design and manufacturing, other factors related to the operation
should be considered: unforeseen voltage spikes, electrolyte decomposition,
effects of applying excessive voltage, and cell performance decay. Electrolyte
decomposition is caused by continued operation at potentials greater than
the acknowledged limits of the dielectric or the electrolyte’s thermodynamic
limitations. Excessive voltage can lead to gas production, mechanical swelling, and cell bursting. Finally decay in cell performance through a reduction
in capacitance and increase in equivalent series resistance should also be
considered in relation to sustainable energy and power supplies to application devices.
5.2.2 Frequency Response
The frequency response of an electrochemical capacitor is dependent upon
its measured time constant τ (τ = R ESR C), which represents the time required
to achieve 63.2% of the full charge from a noncharged state or discharge
32.8% of its charge from a fully charged state. Generally, the frequency
response ranges from milliseconds to seconds, depending on the cell configuration, electrode materials, and electrolyte that all contribute to the equivalent series resistance and capacitance of a cell. Minimizing the resistances
and maximizing the capacitance can increase the efficiency and the power
rating, allowing an ES to play a more critical role in pulse power applications
that require rapid responses with minimal losses of power.
5.2.3 Lifetime and Cycle Charging
Theoretically, the lifetime of an ES is unlimited because no final event indicates that it is dead. However, the continuous charging and discharging of
an ES at a constant current can actually result in exponential decay of the
capacitance, leading to an increase in internal resistance. The end of an ES
life cycle is defined as maximum acceptable loss in relative capacitance. The
life cycle of an ES (Figure 5.1) details the initial dramatic loss generally found
in all cycle testing, a steady linear decline in relative capacitance over time,
Electrochemical Supercapacitor Design, Fabrication, and Operation
power supplies or hybrid electric vehicles, contains numerous ~2.7 V single
cells in series.
In ES stack design and manufacturing, maximizing the safe operating
voltage of each cell is a priority, along with reducing costs by minimizing
material requirements, using cost-effective materials, and simplifying manufacturing processes. In general, a wide ES operating voltage improves the
efficiency of storing energy and supplying power. For example, electronic
systems generally possess a threshold for a minimum operating voltage that
directly relates to the usable power stored in an electrochemical capacitor
cell. At the current technological state, utilization of 75% of the stored energy
can be achieved when an ES is discharged from an initial charge potential
V initial to one half of V initial .
In ES design and manufacturing, other factors related to the operation
should be considered: unforeseen voltage spikes, electrolyte decomposition,
effects of applying excessive voltage, and cell performance decay. Electrolyte
decomposition is caused by continued operation at potentials greater than
the acknowledged limits of the dielectric or the electrolyte’s thermodynamic
limitations. Excessive voltage can lead to gas production, mechanical swelling, and cell bursting. Finally decay in cell performance through a reduction
in capacitance and increase in equivalent series resistance should also be
considered in relation to sustainable energy and power supplies to application devices.
5.2.2 Frequency Response
The frequency response of an electrochemical capacitor is dependent upon
its measured time constant τ (τ = R ESR C), which represents the time required
to achieve 63.2% of the full charge from a noncharged state or discharge
32.8% of its charge from a fully charged state. Generally, the frequency
response ranges from milliseconds to seconds, depending on the cell configuration, electrode materials, and electrolyte that all contribute to the equivalent series resistance and capacitance of a cell. Minimizing the resistances
and maximizing the capacitance can increase the efficiency and the power
rating, allowing an ES to play a more critical role in pulse power applications
that require rapid responses with minimal losses of power.
5.2.3 Lifetime and Cycle Charging
Theoretically, the lifetime of an ES is unlimited because no final event indicates that it is dead. However, the continuous charging and discharging of
an ES at a constant current can actually result in exponential decay of the
capacitance, leading to an increase in internal resistance. The end of an ES
life cycle is defined as maximum acceptable loss in relative capacitance. The
life cycle of an ES (Figure 5.1) details the initial dramatic loss generally found
in all cycle testing, a steady linear decline in relative capacitance over time,