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Electrochemical Supercapacitor Design, Fabrication, and Operation
During ES stack construction and packaging, series stacking is more practical than parallel stacking. In series stacking, individually packaged cells
are serially integrated into a multiunit stack through external metal bars or
soldering the cells onto a PCB board. Alternatively, the uncased jelly rolls
could be compartmentalized to reduce dead space and packaging weight
and increase the intimacy of contacts within the module [10]. However, when
module designs are scaled, the combination of interparticle electrode resistances and parasitic intercell contacts can cause increasingly large pack resistances. Large scale resistances can cause great effects on power performance
[11]. Pack voltage is often tailored by creating external lateral contacts that
also affect resistances. During use, interconnected cells begin to experience
non-uniform charge distribution [12]. These charge distributions can be mitigated by using active and passive balancing components discussed later in
this chapter.
5.4.2 Utilizing Bipolar Design
Use of bipolar electrodes to form an ES stack is shown in Figure 5.9. The
bipolar arrangement can effectively minimize the volume of the stack and
circumvent the use of additional materials and external connections. In addition, the intimate surface level connection can help overcome macroscopic
resistances generated from solder joints, long interconnects, and tabs that
contact only part of the collector foil. The reduction in packing material (grid
weight) for a module also improves cell performance.
Bipolar electrodes are commonly used in fuel cell, battery, and ES applications. The construction of bipolar electrodes requires the use of a highly
conductive paper or plate substrate (e.g. titanium, carbon fiber paper, etc.).
During electrode preparation, one side of the conductive bipolar electrode
sheet is attached to an electroactive layer by pressing a previously prepared
electrode film or depositing an active layer. The other side of this sheet is
attached by another electrode layer. In the stack, one side of the bipolar electrode acts as the anode of a cell and the other side acts as the cathode of
the adjacent cell (Figure 5.9b). It is very important that no ion conductivity
through the bipolar plate is possible in order to prevent short circuits. A gasket, sealant, or non-conducting porous material is placed between each bipolar plate to act as a separator to avoid short circuiting of the cells. An external
pressure is then applied to the outer casing of the cell to improve material
contact and reduce resistances.
Figure 5.10 shows a series sequence of bipolar plates. Each plate has a
double-layer capacitance on both surfaces. However, each side maintains a
polarity opposite that of the electrolyte solution. The spaces between each
bipolar plate are filled with an electrolyte in liquid or hydrated solid polymer form. From the current collecting plate E 1 to the first side of bipolar
plate A 1 , a voltage drop occurs to represent the voltage of the first cell in
the three-cell stack.
Electrochemical Supercapacitor Design, Fabrication, and Operation
During ES stack construction and packaging, series stacking is more practical than parallel stacking. In series stacking, individually packaged cells
are serially integrated into a multiunit stack through external metal bars or
soldering the cells onto a PCB board. Alternatively, the uncased jelly rolls
could be compartmentalized to reduce dead space and packaging weight
and increase the intimacy of contacts within the module [10]. However, when
module designs are scaled, the combination of interparticle electrode resistances and parasitic intercell contacts can cause increasingly large pack resistances. Large scale resistances can cause great effects on power performance
[11]. Pack voltage is often tailored by creating external lateral contacts that
also affect resistances. During use, interconnected cells begin to experience
non-uniform charge distribution [12]. These charge distributions can be mitigated by using active and passive balancing components discussed later in
this chapter.
5.4.2 Utilizing Bipolar Design
Use of bipolar electrodes to form an ES stack is shown in Figure 5.9. The
bipolar arrangement can effectively minimize the volume of the stack and
circumvent the use of additional materials and external connections. In addition, the intimate surface level connection can help overcome macroscopic
resistances generated from solder joints, long interconnects, and tabs that
contact only part of the collector foil. The reduction in packing material (grid
weight) for a module also improves cell performance.
Bipolar electrodes are commonly used in fuel cell, battery, and ES applications. The construction of bipolar electrodes requires the use of a highly
conductive paper or plate substrate (e.g. titanium, carbon fiber paper, etc.).
During electrode preparation, one side of the conductive bipolar electrode
sheet is attached to an electroactive layer by pressing a previously prepared
electrode film or depositing an active layer. The other side of this sheet is
attached by another electrode layer. In the stack, one side of the bipolar electrode acts as the anode of a cell and the other side acts as the cathode of
the adjacent cell (Figure 5.9b). It is very important that no ion conductivity
through the bipolar plate is possible in order to prevent short circuits. A gasket, sealant, or non-conducting porous material is placed between each bipolar plate to act as a separator to avoid short circuiting of the cells. An external
pressure is then applied to the outer casing of the cell to improve material
contact and reduce resistances.
Figure 5.10 shows a series sequence of bipolar plates. Each plate has a
double-layer capacitance on both surfaces. However, each side maintains a
polarity opposite that of the electrolyte solution. The spaces between each
bipolar plate are filled with an electrolyte in liquid or hydrated solid polymer form. From the current collecting plate E 1 to the first side of bipolar
plate A 1 , a voltage drop occurs to represent the voltage of the first cell in
the three-cell stack.