Basic Electronics 16 – Supercapacitors

In previous tutorials, we discussed how to work with a capacitor, the characteristics of a capacitor, various types of capacitors, and selecting a capacitor for a particular circuit. As we learned, typical commercial capacitors have capacitance in the Picofarad, Nanofarad, or Microfarad range. The maximum capacitance these capacitors can provide is 1 Farad. If higher capacitance is required, the capacitors will need to be quite large, which may or may not fit into typical electronic circuits.

Enter the supercapacitor. These electrochemical type capacitors are small and can offer capacitance in tens, hundreds or even thousands of Farads. They not only store a large amount of charge, but can also go through several thousand charge and discharge cycles without any wear and tear. That's why these capacitors, also known as ultracapacitors, are useful in many emerging technologies, such as hybrid vehicles, active filters, renewable energy, UPS, smartphones and portable electronic devices.

What they do
Supercapacitors are used to store a large amount of charge as an electrostatic field. Just like electrolytic capacitors, these capacitors also use liquid or solid electrolytes. However, the way they store cargo is totally different. In typical capacitors, charge is stored due to polarization of the dielectric material. In supercapacitors, the electrolyte does not serve as a dielectric. It only delivers charge carriers to the electrodes. Instead, charge is stored by the accumulation of opposing charge carriers at the electrodes.

The electrodes of these capacitors are made of porous activated carbon or carbon nanotubes, which are capable of attracting a large amount of charge to themselves. A minimum distance separates the electrodes and has a separator between them to avoid short circuits between the porous electrodes. The use of activated carbon as electrodes and a minimum distance between electrodes allows these capacitors to store a large amount of charge in a small size.

Construction
Supercapacitors are constructed similarly to electrolytic capacitors. They have two electrodes made of porous coating of active carbon or carbon nanotubes. The coating is applied to metal sheets (usually aluminum) that serve as current collectors. Current collectors covered with electrodes are immersed in an electrolyte.

The electrolyte can be liquid or solid. In most ultracapacitors, solid electrolytes are preferred due to higher terminal voltage. The solid electrolyte is generally a solvent mixed with conductive salts. Typically, acetonitrile or propylene carbonate is used as a solvent and tetraalkylammonium or lithium salts as solutes. The electrode-coated current collectors are separated by a separator (paper membrane) that is transparent to the charge carriers but prevents direct short-circuiting between the electrodes. Due to the double-sided electrode coating of the current collectors, these capacitors are also called Electrical Double Layer Capacitors (EDLC).

The highly porous nature of the electrode material allows these capacitors to attract a large number of charge carriers from the electrolyte. Due to the use of activated carbon, the effective surface area between the current collectors increases many times over. The internal resistance (ESR) of the capacitor depends on the electrolyte. The lower the resistance offered by the electrolyte, the greater the power density of the capacitor.

Supercapacitors generally have a very low voltage rating that can range from 1V to 3V. The following equation gives the electrical energy stored by a supercapacitor:

P = V ² /4R
Where,
P is the power stored by the Super Capacitor,
V is the applied voltage (or voltage rating),
R is the internal resistance (ESR) of the capacitor

How they work
When a potential difference is applied across the terminals of a supercapacitor, the electrodes begin to attract opposite charge carriers from the electrolyte. Positive ions accumulate in the negative connection and negative ions accumulate in the positive connection. Charge carriers are stored on current collector plates. Due to the accumulation of opposite charges in the current collectors, an electrostatic field is created between them. The charging current flows through the capacitor until the electrostatic field between the current collectors is equal and opposite to the applied voltage. Charge carriers are retained by current collectors until the applied voltage decreases or changes polarity.

Whenever the applied voltage decreases, a proportional number of charge carriers are passed back to the electrolyte from the current collectors. During this process, an equivalent current flows through the capacitor in the reverse direction. When the polarity changes, the supercapacitor goes through a similar charge and discharge cycle.

You see that the supercapacitor, despite its electrochemical construction, still stores charge in the form of an electrostatic field. It works exactly like any other capacitor. This is why, despite their battery-like construction, supercapacitors are classified as capacitors and not batteries. Compared to batteries, supercapacitors can undergo several thousand charge and discharge cycles. Therefore, they can serve as an excellent source of charge or reserve power in battery-operated circuits.

Practical supercapacitors
Supercapacitor cells have a very low terminal voltage that can range from 1V to 3V. By connecting supercapacitor cells in series, their voltage rating can be multiplied. Similarly, parallel connection of supercapacitor cells multiplies the effective capacitance. As a result, supercapacitors are generally used as an array of cells where they are connected in series along rows and in parallel along columns. The following equation provides the final voltage rating of the package:

V = N * V _Cell
Where,
V is the effective terminal voltage of the package
N is the number of rows or number of supercapacitor cells connected in series in each column
V _Cell is the terminal voltage of individual supercapacitor cells

The following equation gives the effective capacitance of the package:

C = (M/N)*C _Cell
Where,
C is the effective capacitance
M is the number of columns or number of supercapacitor cells connected in parallel in each row
N is the number of rows or number of supercapacitor cells connected in series in each column
C _Cell is the capacitance of individual supercapacitor cells

Benefits
Supercapacitors have the following notable advantages over other capacitors and batteries:

• Capable of storing a large amount of energy in the form of an electrostatic field.
• High power density and compact size, which makes them suitable for charge storage in typical electronic circuits.
• Ability to charge and discharge in a short space of time and can be used to meet frequent spikes in power demand and can provide large bursts of power for short periods.
• They do not involve electrochemical reactions and, therefore, do not present operational wear and increase useful life. They can be used hundreds of thousands of times without needing to be replaced.

Forms
Due to their charge storage capacity, small size, and fast charging and recharging, supercapacitors have found applications in many emerging technologies. One of the major domains where supercapacitors have eventually reserved a place is the transportation industry. In electric vehicles, supercapacitors are used in braking systems to store EMF produced by DC motors. The charge stored by supercapacitors can be applied to power the vehicle's electrical systems and recharge batteries. In electric vehicles, supercapacitors can also serve as a power reserve for batteries, so smaller batteries need to be installed in them. Hybrid vehicles (which completely turn off the engine when stopping) use supercapacitors to restart the engine after each stop.

Another useful application of supercapacitors is in traction vehicles. Traction vehicles go through multiple phases of acceleration, cruising and deceleration. Supercapacitors can be used in such vehicles to deal with voltage fluctuations and operational power losses.

Today, supercapacitors are generally used along with display units of smartphones, laptops and computers to turn on the screen in a short time. They are also used in memory devices and power backups.

Supercapacitors are now used in many other application spaces where urgent power backups or immediate power surges are required. Combining supercapacitors with batteries can enable smaller, more compact electrical power sources.

In the next tutorial, we will discuss inductors.