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Electrochemical Supercapacitors for Energy Storage and Delivery
or voltage. For example, some adverse chemical activities such as electrode
and electrolyte decomposition can be enhanced by temperature or voltage
increases and can increase cell resistance.
Additionally, the metal- and oxygen-containing surface-functional groups
on the active carbon are also partially responsible for performance degradation from participating in electrolyte decomposition reactions that ultimately
reduce the microporosity of the electrode. For example, in the presence of oxy–
–
genated and/or residual water, the decomposition of BF 4 anion to BF x O y ions
can occur. Solid products formed from the redox decomposition of the electrolyte can block the pores on carbon particles, causing a loss in capacitance.
Decomposition products also block pores within the cell separator, causing
an increase in electrolyte resistance. Water impurity in organic electrolytes is
believed to be partially responsible for the macroscopic effects and capacitance
fading observed during cell aging. In addition, interaction of the electrode functional groups with organic electrolyte TEABF 4 (tetraethylammonium tetrafluoroborate) in acetonitrile also causes capacitance fading and increased resistance.
In general, the positive electrode (anode) is more susceptible to loss in surface area. Aging studies found that fluoride could be bonded covalently to
the anodic carbon support. The nitrogen species including pyridinic (-C =
N-C-) and amine (C-NH 2 ) moieties can also bond covalently to the graphitic
structure of the anode, facilitating the polymerization of the acetonitrile solvent and causing aging of the anode [21,22].
To study aging, a high operating voltage above the nominal rate of 2.5 to
2.7 V was employed using quaternary ammonium salts in propylene carbonate (PC) or acetonitrile. Gas degradation products at both electrodes from an
elevated voltage (2.6 to 4 V) are CO, CO 2 , ethene, propene, and H 2 , resulting
in pressure increase, then losses in electrolyte ion and electrode cohesion.
Evolution of CO 2 at a nominal voltage (2.5 V) and rated temperature limit
(70°C) comes from the reaction of acetonitrile and water.
The electrode kinetics and effects on the ES electrochemical performance
have also been studied. At an extremely low kinetic rate, capacitance loss
accompanied by an increase in series resistance could occur, leading to
aging. This aging is accelerated at both high temperatures and voltages.
Understanding the mechanisms of ES cell aging is important in developing mitigation strategies to improve device lifetimes. Furthermore, the
knowledge of cell aging can held find ways to increase the temperature or
charge conditions that currently limit the use of ES systems.
5.7 Self Discharging
Self-discharging phenomena are common in all ESs but their origins are
not well understood. Self discharging can reduce performance in terms of
Electrochemical Supercapacitors for Energy Storage and Delivery
or voltage. For example, some adverse chemical activities such as electrode
and electrolyte decomposition can be enhanced by temperature or voltage
increases and can increase cell resistance.
Additionally, the metal- and oxygen-containing surface-functional groups
on the active carbon are also partially responsible for performance degradation from participating in electrolyte decomposition reactions that ultimately
reduce the microporosity of the electrode. For example, in the presence of oxy–
–
genated and/or residual water, the decomposition of BF 4 anion to BF x O y ions
can occur. Solid products formed from the redox decomposition of the electrolyte can block the pores on carbon particles, causing a loss in capacitance.
Decomposition products also block pores within the cell separator, causing
an increase in electrolyte resistance. Water impurity in organic electrolytes is
believed to be partially responsible for the macroscopic effects and capacitance
fading observed during cell aging. In addition, interaction of the electrode functional groups with organic electrolyte TEABF 4 (tetraethylammonium tetrafluoroborate) in acetonitrile also causes capacitance fading and increased resistance.
In general, the positive electrode (anode) is more susceptible to loss in surface area. Aging studies found that fluoride could be bonded covalently to
the anodic carbon support. The nitrogen species including pyridinic (-C =
N-C-) and amine (C-NH 2 ) moieties can also bond covalently to the graphitic
structure of the anode, facilitating the polymerization of the acetonitrile solvent and causing aging of the anode [21,22].
To study aging, a high operating voltage above the nominal rate of 2.5 to
2.7 V was employed using quaternary ammonium salts in propylene carbonate (PC) or acetonitrile. Gas degradation products at both electrodes from an
elevated voltage (2.6 to 4 V) are CO, CO 2 , ethene, propene, and H 2 , resulting
in pressure increase, then losses in electrolyte ion and electrode cohesion.
Evolution of CO 2 at a nominal voltage (2.5 V) and rated temperature limit
(70°C) comes from the reaction of acetonitrile and water.
The electrode kinetics and effects on the ES electrochemical performance
have also been studied. At an extremely low kinetic rate, capacitance loss
accompanied by an increase in series resistance could occur, leading to
aging. This aging is accelerated at both high temperatures and voltages.
Understanding the mechanisms of ES cell aging is important in developing mitigation strategies to improve device lifetimes. Furthermore, the
knowledge of cell aging can held find ways to increase the temperature or
charge conditions that currently limit the use of ES systems.
5.7 Self Discharging
Self-discharging phenomena are common in all ESs but their origins are
not well understood. Self discharging can reduce performance in terms of