116
6 Supercapacitors
Supercapacitors are another type of energy storage material, and are often grouped
with batteries when describing the most promising applications for modern, green
energy storage solutions. Batteries are known to have a high specific energy, but low
power density. In contrast, supercapacitors provide a lower energy density but much
higher power density. Although the two are often posed as competitors, batteries, and
supercapacitors but may also be coupled to obtain the properties of both [114]. An
important and desirable aspect of supercapacitors is they can be charged and discharged rapidly, as they use electrostatics to store charge, rather than internal reactions of ions between electrodes, to obtain an immediately available energy supply.
Furthermore, as there is no chemical reaction to consider, they are theoretically
nearly infinitely cycleable with little degradation. An example lies in the use of allelectric hybrid buses. The supercapacitors can be used to quickly accelerate the buses
and recharge upon braking (which happens frequently on bus routes), but do not have
enough energy to power the bus alone. Hence, batteries provide energy storage for
the bulk of the ride. Also, buses and vehicles that are powered only by supercapacitors, called “capa” vehicles, are also possible. These contain supercapacitors under
the seats, can be partially powered by braking and while stopped at bus stops via
power collectors, and are less expensive than lithium ion batteries, therefore holding
great promise for use in situations where a route is known and stops are planned [115].
In the basic design of any capacitor, two conductors are separated by a dielectric
that can be polarized in the presence of an electric field. In a simple dielectric capacitor, metal plates are separated by a dielectric. When charged, the positive charge
moves to one of the metal plates, while the negative charges move to the other,
creating an electric field that results in the alignment of dipoles in the dielectric. The
measured capacitance for typical solid-state electronics is on the order of picofarads
to microfarads, limiting the functionality of these for energy storage. Since the
capacitance is an extrinsic property that depends on the amount of material, higher
surface area would yield a higher concentration of charge, allowing for smaller
devices. Supercapacitors operate using the adsorption/desorption of ions in an electrolyte that is in contact with much higher surface area electrodes (i.e. activated
carbon), but on a grander scale, resulting in capacitance values on the order of
Farads, which is one million or billion times larger than standard dielectric capacitors. Yet again, nanotechnology has been instrumental in advancing the utility of
capacitors, as the nano-featured surfaces provide the requisite high surface area.
The individual, metal-coated porous plates are soaked in an electrolyte and placed
next to one another with a thin insulator between. Upon charging, the two sides
develop a charge separation, and the electrolytes polarize with the charges aligned
on either side. Because both dielectric layers may be as thin as one molecular layer,
such electrolytes are often referred to as double-layer dielectrics. The electrolytes
used will change the operating characteristics of the supercapacitor, including the
voltage and capacitance. Due to the high concentration of charges, aqueous solutions such as of sulfuric acid have traditionally been used in the past, but the aqueous phase limits the voltage possibilities.
C. A. Bauer
6 Supercapacitors
Supercapacitors are another type of energy storage material, and are often grouped
with batteries when describing the most promising applications for modern, green
energy storage solutions. Batteries are known to have a high specific energy, but low
power density. In contrast, supercapacitors provide a lower energy density but much
higher power density. Although the two are often posed as competitors, batteries, and
supercapacitors but may also be coupled to obtain the properties of both [114]. An
important and desirable aspect of supercapacitors is they can be charged and discharged rapidly, as they use electrostatics to store charge, rather than internal reactions of ions between electrodes, to obtain an immediately available energy supply.
Furthermore, as there is no chemical reaction to consider, they are theoretically
nearly infinitely cycleable with little degradation. An example lies in the use of allelectric hybrid buses. The supercapacitors can be used to quickly accelerate the buses
and recharge upon braking (which happens frequently on bus routes), but do not have
enough energy to power the bus alone. Hence, batteries provide energy storage for
the bulk of the ride. Also, buses and vehicles that are powered only by supercapacitors, called “capa” vehicles, are also possible. These contain supercapacitors under
the seats, can be partially powered by braking and while stopped at bus stops via
power collectors, and are less expensive than lithium ion batteries, therefore holding
great promise for use in situations where a route is known and stops are planned [115].
In the basic design of any capacitor, two conductors are separated by a dielectric
that can be polarized in the presence of an electric field. In a simple dielectric capacitor, metal plates are separated by a dielectric. When charged, the positive charge
moves to one of the metal plates, while the negative charges move to the other,
creating an electric field that results in the alignment of dipoles in the dielectric. The
measured capacitance for typical solid-state electronics is on the order of picofarads
to microfarads, limiting the functionality of these for energy storage. Since the
capacitance is an extrinsic property that depends on the amount of material, higher
surface area would yield a higher concentration of charge, allowing for smaller
devices. Supercapacitors operate using the adsorption/desorption of ions in an electrolyte that is in contact with much higher surface area electrodes (i.e. activated
carbon), but on a grander scale, resulting in capacitance values on the order of
Farads, which is one million or billion times larger than standard dielectric capacitors. Yet again, nanotechnology has been instrumental in advancing the utility of
capacitors, as the nano-featured surfaces provide the requisite high surface area.
The individual, metal-coated porous plates are soaked in an electrolyte and placed
next to one another with a thin insulator between. Upon charging, the two sides
develop a charge separation, and the electrolytes polarize with the charges aligned
on either side. Because both dielectric layers may be as thin as one molecular layer,
such electrolytes are often referred to as double-layer dielectrics. The electrolytes
used will change the operating characteristics of the supercapacitor, including the
voltage and capacitance. Due to the high concentration of charges, aqueous solutions such as of sulfuric acid have traditionally been used in the past, but the aqueous phase limits the voltage possibilities.
C. A. Bauer