Supercapacitor Supercap or Ultracapacitor

- an overview or tutorial about the supercapacitor or supercap, also known as an ultracapacitor or double layer capacitor which provides a very high level of capacitance.

As the name indicates, the supercapacitor is a specialised form of capacitor with an exceedingly high level of capacitance.

These devices which are also known as supercaps or ultracapacitors may have capacitance levels up to several thousand farads and as a result they are half way between a capacitor and a battery. These capacitors may also be known as double-layer capacitors as a result of the way in which they are constructed.

An image of a typical supercapacitor showing the package of a leaded supercapacitor

Supercapacitor applications

These supercapacitors or ultracapacitors are used in a number of applications. Their very high storage and the high current capability enable them to be ideally suited for a number of instances.

Initially their most widespread use was in memory battery back-up. Here they were able to maintain their charge when a system as running, and then deliver the stored charge to maintain the memory in periods when the main power was interrupted. They are also used in UPS, Uninterruptible Power Supply systems. Here they are able to provide high levels of power for a short time. They are typically used in tandem with a complementary power source to supply energy in peak power demand conditions, reducing strain on the primary source and extending its usable life.

An image showing a supercapacitor and a double layer capacitor with a coin to indicate the size

The high current capability of these supercaps or ultracapacitors has also enabled them to be used in various motor / power applications. They have been used in electric power trains and also to provide peak load enhancement within hybrid vehicles. Here they can be used in regenerative braking systems, storing the energy reclaimed during braking, and then releasing it to power an electric motor later.

Supercapacitor vs battery comparison

The supercapacitor and some battery technologies compete for some applications. It is worth taking a look at the comparison between a typical supercapacitor and a Lithium Ion high capacity battery technology used today. Lithium ion technology has been taken for the comparison because it is the highest density battery technology in widespread use today.

The comparisons made are only for approximate evaluation and any decision should be based on exact figure obtained from manufacturers for devices being considered because technology in both areas is advancing swiftly.

Comparison between Super Capacitor and Lithium Ion Battery
Parameter Supercapacitor Lithium-ion
Cell voltage 2.3 - 2.7V 3.6V
Charge time 1 - 10 seconds 10 - 60 minutes
Lifetime (charge / discharge cycles) ~1 million 500 - 3000
Specific energy (Wh/kg) ~5 100 - 200
Specific power (W/kg) up to 10 000 1000 - 3000
Output voltage maintenance Poor Good
Safety Relatively safe when abused Less safe and have been known to explode on rare occasions
Cost per Wh ~ 20 units ~1 unit
Service life ~10 years ~5 years
Operating temperature range ~ -40 to +65°C ~ 0 to +40°C

Supercapacitor technology basics

The technology behind supercapacitors is different to that of an ordinary capacitor. It has to adopt a different approach to enable the ultra-high values of capacitance to be attained.

There are several types of supercap or ultracapacitor technology that can be used but the most widely adopted is known as the double-layer capacitor, DLC. This form of the technology is carbon-based and it has an organic electrolyte that is easy to manufacture.

Like traditional capacitors, supercapacitors have two metal plates. These plates are coated with activated carbon which is a sponge-like porous material. These plates are immersed in an electrolyte which contains positive and negative ions. One carbon-coated plate, or electrode, is positive, and the other is negative. During charging, ions from the electrolyte accumulate on the surface of each carbon-coated plate.

The basic double layer capacitor cell of a supercapacitor showing the two electrodes with their associated capacitor layers and the electrolyte layer.
Basic supercapacitor double layer capacitor cell

As charging occurs, the carbon electrodes have two layers of charge coating their surfaces and this gives rise to the name double layer capacitors. The distance between the two charge layers at the electrode is exceedingly small and this means that very high capacitance levels are achievable. Also as there are charge layers at each electrode, in effect a supercapacitor is actually two capacitors in series, one at each electrode

There are two types of double layer capacitor resulting from different charge storage mechanisms:

  • Electrical double-layer capacitor:   An EDLC stores energy in the double-layer at the electrode/electrolyte interface. In this type of capacitor, the electrode material used for the construction of the cell for the former is mainly carbon material.
  • Electrochemical double layer capacitor or super/pseudo-capacitor :   supercapacitor sustains a Faradic reaction between the electrode and the electrolyte in a suitable potential window. In this type of supercapacitor the electrode material consists of either transition metal oxides or mixtures of carbon and metal oxides/polymers

For both types of capacitor, the electrolytes can be either aqueous or non-aqueous depending on the mode of construction of capacitor cell.

It is for this reason for supercapacitors and double layer capacitors may be marketed separately using different names. Often the double layer capacitors do not have quite as high a capacitance level.

Supercaps limitations

The supercapacitor or ultracapacitor is widely used for many applications. When using them there are a few points to note:

  • Maximum voltage:   There is a maximum voltage for ultracapacitors. Other types of capacitor can be manufactured to operate at high voltages, but supercapacitors are generally restricted to operating voltages in the region of 2.5 - 2.7V. It is possible to manufacture them for operation above 2.8V but it is found that the operational life is reduced.
  • Series operation:   In order to achieve higher operating voltages for supercapacitors, they can be placed in series. This reduces the total capacitance as is the case with any capacitors placed in series. Also if more than three capacitors need to be placed in series then it is necessary to implement voltage balancing techniques. As the leakage currents through the capacitors are likely to be different, the voltage split across the capacitors will not be equal and one or more may enter and over-voltage position.
  • Self-discharge:   Self discharge of supercapacitors can be an issue in some circumstances. It results from the electrolyte used and it means that the energy stored can decrease by 50% or more within about a month or so. By comparison a nickel based battery (NiCd or NiMH) self-discharges by about 10% or more a month and a Li-ion battery by about 5% a month.

By Ian Poole

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