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Electrochemical Supercapacitor Design, Fabrication, and Operation
two materials and reduce series resistances. Normally, acid or anode etching
is used to treat the collector surface.
As noted earlier, bipolar electrode stacking arrangements have a volumetric
efficiency. Unfortunately, a drawback for bipolar electrode arrangements is
that the heat dispersion is more difficult than occurs with an external connection due to the internal I 2 R heat generation within the stack. If this heat is not
effectively managed, over-heating will lead to cell degradation. Further, cooling surface area is low within the bipolar stack, making it difficult to incorporate passive cooling without high thermal conduction properties [15]. This
concern is more significant in large-scale supercapacitor packs. Therefore an
additional cooling system is needed, but will cause a parasitic loss of stack
power. In addition, for a bipolar electrode-based ES stack, proper sealing of
the stack is crucial to circumvent any form of electrolyte leakage that ultimately creates undesirable shunt currents that cause self discharge behavior.
5.5 Voltage Cell Balancing
In an ES stack containing a number of single cells, it is impossible to have the
same voltages in all cells when operating under a load. Some cells have high
voltages and others have low voltages. This creates an unbalanced stack and
results in low efficiency. In general, low cell voltage is caused by high internal resistance and high voltage is caused by low capacitance. For an ES stack,
the damage from high cell voltage is more severe than that caused by low cell
voltage because it could lead to gas evolution and stack rupture
If all cells in a stack are in series, they will experience an equal current. The
cell voltage is relatively dependent on capacitance and charging current, so
incorporating a single cell with a smaller capacitance will lead to a higher voltage across the cell. A cell with a larger than average capacitance will experience
a smaller voltage. This will become even worse during stack charging, as the
voltage distributed across a stack is initially dependent on the capacitance. The
cells having lower capacitances will experience voltages higher than the limited
ratings of individual cells within the stack, leading to aging and performance
degradation. Unequal capacitances of cells can arise from several factors aside
from manufacturing variances including cell aging and temperature gradients.
It is important in stack design to balance voltages to prevent unequal voltage
distributions. This requires consistency in the manufacturing of single cells to
make every cell have the same capacitance and internal resistance or make the
capacitance and internal resistance as close as possible. An engineering concern is that the material mass and mode of construction should be equivalent
in each manufactured cell. To further balance the voltage distribution to avoid
damaging effects, two types of voltage equalization circuits developed for this
purpose are passive balancing and active balancing.
Electrochemical Supercapacitor Design, Fabrication, and Operation
two materials and reduce series resistances. Normally, acid or anode etching
is used to treat the collector surface.
As noted earlier, bipolar electrode stacking arrangements have a volumetric
efficiency. Unfortunately, a drawback for bipolar electrode arrangements is
that the heat dispersion is more difficult than occurs with an external connection due to the internal I 2 R heat generation within the stack. If this heat is not
effectively managed, over-heating will lead to cell degradation. Further, cooling surface area is low within the bipolar stack, making it difficult to incorporate passive cooling without high thermal conduction properties [15]. This
concern is more significant in large-scale supercapacitor packs. Therefore an
additional cooling system is needed, but will cause a parasitic loss of stack
power. In addition, for a bipolar electrode-based ES stack, proper sealing of
the stack is crucial to circumvent any form of electrolyte leakage that ultimately creates undesirable shunt currents that cause self discharge behavior.
5.5 Voltage Cell Balancing
In an ES stack containing a number of single cells, it is impossible to have the
same voltages in all cells when operating under a load. Some cells have high
voltages and others have low voltages. This creates an unbalanced stack and
results in low efficiency. In general, low cell voltage is caused by high internal resistance and high voltage is caused by low capacitance. For an ES stack,
the damage from high cell voltage is more severe than that caused by low cell
voltage because it could lead to gas evolution and stack rupture
If all cells in a stack are in series, they will experience an equal current. The
cell voltage is relatively dependent on capacitance and charging current, so
incorporating a single cell with a smaller capacitance will lead to a higher voltage across the cell. A cell with a larger than average capacitance will experience
a smaller voltage. This will become even worse during stack charging, as the
voltage distributed across a stack is initially dependent on the capacitance. The
cells having lower capacitances will experience voltages higher than the limited
ratings of individual cells within the stack, leading to aging and performance
degradation. Unequal capacitances of cells can arise from several factors aside
from manufacturing variances including cell aging and temperature gradients.
It is important in stack design to balance voltages to prevent unequal voltage
distributions. This requires consistency in the manufacturing of single cells to
make every cell have the same capacitance and internal resistance or make the
capacitance and internal resistance as close as possible. An engineering concern is that the material mass and mode of construction should be equivalent
in each manufactured cell. To further balance the voltage distribution to avoid
damaging effects, two types of voltage equalization circuits developed for this
purpose are passive balancing and active balancing.