58
Electrochemical Supercapacitors for Energy Storage and Delivery
Consequently, the electrode behaves like a capacitor and only capacitive
current (no faradic current) flows upon a change of potential in a certain
electrode potential range. The electrode [carbon/1.0 M NaCl (aq) interface]
mentioned above can behave as an ideal polarized electrode but only within
the double-layer range from –0.6 to 0.2 V versus NHE.
Likewise, ideal non-polarizable electrodes are not polarizable. The potential of the ideal non-polarizable electrode will not change from its equilibrium potential even with the application of a large current density. The
electrode reaction is extremely fast with an almost infinite exchange current
density. For example, using an NHE electrode with a large Pt surface (Pt
black) and an electrochemical current as high as several amperes per geometric square centimeter will not result in a significant change in its electrode potential. However, most electrode–electrolyte interfaces are between
the ideal polarizable and non-polarizable electrodes.
In a practical application of a double-layer supercapacitor, the wider the
double-layer, the higher the capacity for charge storage. However, the double-layer potential range or window is strongly dependent on the electrode
material, the electrolyte, and the solvent used. The most practical electrode
materials in supercapacitors are carbon-based and have almost ideal polarizable potential windows in an electrolyte solution. However, their surface
reversible redox reactions to produce pseudocapacitance are actually beneficial by contributing to the capacity of charge storage.
It is important to choose an electrolyte with a wide electrochemically stable
range. For a solvent, the selection seems difficult due to its intrinsic electrochemical stability. For example, for an aqueous solution, the electrochemical
disassociation window of water is around 1.23 V at room temperature. If
water is used as a supercapacitor electrolyte solvent, the maximum cell voltage will be around 1.23 V; if acetonitrile is the solvent, the electrode potential
window is around 2.0 V; with an ion liquid, the electrode potential window
can be as high as 4.0 V. Therefore, different solvents have different potential
windows. Table 2.2 lists several common solvents and their potential windows for supercapacitors.
2.4 Capacitance of Porous Carbon Materials
The differential capacitance for a smooth electrode–electrolyte interface
was discussed earlier in this chapter. The unit of differential capacitance
is expressed as farads per square meter (F.m –2 ) or microfarads per square
meter (μF.cm –2 ). Strictly speaking, this is called the capacitance density. If the
capacitance density is C dl in F.m –2 and the electrode surface area is defined
as A in square meters, the capacitance of the entire electrode is C dl A in F. If
Electrochemical Supercapacitors for Energy Storage and Delivery
Consequently, the electrode behaves like a capacitor and only capacitive
current (no faradic current) flows upon a change of potential in a certain
electrode potential range. The electrode [carbon/1.0 M NaCl (aq) interface]
mentioned above can behave as an ideal polarized electrode but only within
the double-layer range from –0.6 to 0.2 V versus NHE.
Likewise, ideal non-polarizable electrodes are not polarizable. The potential of the ideal non-polarizable electrode will not change from its equilibrium potential even with the application of a large current density. The
electrode reaction is extremely fast with an almost infinite exchange current
density. For example, using an NHE electrode with a large Pt surface (Pt
black) and an electrochemical current as high as several amperes per geometric square centimeter will not result in a significant change in its electrode potential. However, most electrode–electrolyte interfaces are between
the ideal polarizable and non-polarizable electrodes.
In a practical application of a double-layer supercapacitor, the wider the
double-layer, the higher the capacity for charge storage. However, the double-layer potential range or window is strongly dependent on the electrode
material, the electrolyte, and the solvent used. The most practical electrode
materials in supercapacitors are carbon-based and have almost ideal polarizable potential windows in an electrolyte solution. However, their surface
reversible redox reactions to produce pseudocapacitance are actually beneficial by contributing to the capacity of charge storage.
It is important to choose an electrolyte with a wide electrochemically stable
range. For a solvent, the selection seems difficult due to its intrinsic electrochemical stability. For example, for an aqueous solution, the electrochemical
disassociation window of water is around 1.23 V at room temperature. If
water is used as a supercapacitor electrolyte solvent, the maximum cell voltage will be around 1.23 V; if acetonitrile is the solvent, the electrode potential
window is around 2.0 V; with an ion liquid, the electrode potential window
can be as high as 4.0 V. Therefore, different solvents have different potential
windows. Table 2.2 lists several common solvents and their potential windows for supercapacitors.
2.4 Capacitance of Porous Carbon Materials
The differential capacitance for a smooth electrode–electrolyte interface
was discussed earlier in this chapter. The unit of differential capacitance
is expressed as farads per square meter (F.m –2 ) or microfarads per square
meter (μF.cm –2 ). Strictly speaking, this is called the capacitance density. If the
capacitance density is C dl in F.m –2 and the electrode surface area is defined
as A in square meters, the capacitance of the entire electrode is C dl A in F. If