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Electrochemical Supercapacitor Design, Fabrication, and Operation
energy and power densities. For example, the observed loss in the open circuit voltage of an ES over time after charging is normally caused by self discharging. Even when operating with a load, fast self discharging can cause
a significant power loss. Several factors affect the self-discharging process,
including charging history, chemistry and electrochemistry of the system,
purity of the electrode materials and electrolyte, operating temperature and
voltage, and pore structure. The voltage decay caused by self discharging is
determined by the self-discharging mechanism(s). In general, self discharging can be induced by five processes:
Faradic decomposition reactions of electrolyte solvent — If the charging
voltage is beyond the thermodynamic limitations of the solvent, the solvent
will be electrochemically oxidized or reduced. For example, if water is used
as the solvent, it could be oxidized to O 2 at the positive electrode and reduced
to H 2 at the negative electrode.
Parasitic redox reactions involving impurities of electrode materials
and electrolytes — During the charging process, if the electrode material
contains electrochemically active surface groups, oxidation will occur on the
positive electrode and a reduction will occur on the negative electrode, forming a complete electrochemical reaction. If the charging voltage is beyond
the reversible redox voltage, a faradic charge leakage current can result,
especially when the concentrations of the corresponding redox couples are
high. For metal impurities in carbon materials, a similar mechanism can
also cause self discharging. If the electrolyte contains impurities that are oxidized on the positive electrode or reduced on the negative electrode over the
potential range of ES cell voltage, a faradic charge leakage current will cause
self discharging.
Reduction of dissolved oxygen in electrolyte — Under the driving force
of cell voltage, oxygen dissolved from the air or produced by an overcharged
cell voltage can be reduced to peroxide or water on the negative electrode.
Simultaneously, other oxidizable compounds such as electrode impurities or
H 2 produced by overcharge voltage can be oxidized on the positive electrode,
forming a self-discharging current that consumes the charge stored in the
ES. Although the dissolved oxygen can be removed before cell assembly to
reduce the effect of self discharging, some degree of discharge may remain
due to carbon’s affinity to adsorb oxygen.
Charge redistribution within deep pores of electrode material — A
dominant effect of self discharge can be the redistribution of charge within
the deep pores of an electrode material. When an ES is fully charged, the
deep pores cannot be fully accessed by the charge in a short time. After
charging stops, charge carriers will begin to travel further down the depths
of the pores to develop a more uniform distribution of charge and additional ions will continue to enter the pores. Depending on the branching and
tortuosity of the pore structure and the geometry of the pores, ~50 hr are
required to ensure uniform distribution of the charge along the pore depth.
Electrochemical Supercapacitor Design, Fabrication, and Operation
energy and power densities. For example, the observed loss in the open circuit voltage of an ES over time after charging is normally caused by self discharging. Even when operating with a load, fast self discharging can cause
a significant power loss. Several factors affect the self-discharging process,
including charging history, chemistry and electrochemistry of the system,
purity of the electrode materials and electrolyte, operating temperature and
voltage, and pore structure. The voltage decay caused by self discharging is
determined by the self-discharging mechanism(s). In general, self discharging can be induced by five processes:
Faradic decomposition reactions of electrolyte solvent — If the charging
voltage is beyond the thermodynamic limitations of the solvent, the solvent
will be electrochemically oxidized or reduced. For example, if water is used
as the solvent, it could be oxidized to O 2 at the positive electrode and reduced
to H 2 at the negative electrode.
Parasitic redox reactions involving impurities of electrode materials
and electrolytes — During the charging process, if the electrode material
contains electrochemically active surface groups, oxidation will occur on the
positive electrode and a reduction will occur on the negative electrode, forming a complete electrochemical reaction. If the charging voltage is beyond
the reversible redox voltage, a faradic charge leakage current can result,
especially when the concentrations of the corresponding redox couples are
high. For metal impurities in carbon materials, a similar mechanism can
also cause self discharging. If the electrolyte contains impurities that are oxidized on the positive electrode or reduced on the negative electrode over the
potential range of ES cell voltage, a faradic charge leakage current will cause
self discharging.
Reduction of dissolved oxygen in electrolyte — Under the driving force
of cell voltage, oxygen dissolved from the air or produced by an overcharged
cell voltage can be reduced to peroxide or water on the negative electrode.
Simultaneously, other oxidizable compounds such as electrode impurities or
H 2 produced by overcharge voltage can be oxidized on the positive electrode,
forming a self-discharging current that consumes the charge stored in the
ES. Although the dissolved oxygen can be removed before cell assembly to
reduce the effect of self discharging, some degree of discharge may remain
due to carbon’s affinity to adsorb oxygen.
Charge redistribution within deep pores of electrode material — A
dominant effect of self discharge can be the redistribution of charge within
the deep pores of an electrode material. When an ES is fully charged, the
deep pores cannot be fully accessed by the charge in a short time. After
charging stops, charge carriers will begin to travel further down the depths
of the pores to develop a more uniform distribution of charge and additional ions will continue to enter the pores. Depending on the branching and
tortuosity of the pore structure and the geometry of the pores, ~50 hr are
required to ensure uniform distribution of the charge along the pore depth.