Introduction to Supercapacitors

What are supercapacitors?

Supercapacitors are electronic devices used to store extremely large amounts of electrical charge. They are also known as double-layer capacitors or supercapacitors. Instead of using traditional dielectrics, supercapacitors use two mechanisms to store electrical energy: double-layer capacitance and pseudocapacitance. Double-layer capacitance is electrostatic in origin, while pseudocapacitance is electrochemical, which means that supercapacitors combine the workings of ordinary capacitors with those of ordinary batteries. Capacitances achieved using this technology can be as high as 12,000 F. In comparison, the self-capacitance of the entire planet Earth is only 710 μF, more than 15 million times smaller than the capacitance of a supercapacitor. Ordinary electrostatic capacitors may have a higher maximum operating voltage, and the typical maximum charge voltage of supercapacitors is between 2.5 and 2.7 volts. Supercapacitors are polar devices, which means that they must be connected to the circuit in the right way, just like electrolytic capacitors. The electrical properties of these devices, especially their fast charge and discharge times, are very interesting for some applications, and supercapacitors may completely replace batteries.

Definition of Supercapacitors

A supercapacitor is a specially designed capacitor with a very large capacitance. Supercapacitors combine the properties of capacitors and batteries into one device.

Features

Charging time

Supercapacitors have charging and discharging times comparable to regular capacitors. Due to their low internal resistance, high charging and discharging currents can be achieved. Batteries typically take several hours to reach a fully charged state - a good example is a mobile phone battery, while supercapacitors can reach the same state of charge in less than two minutes.

Specific power

The specific power of a battery or supercapacitor is a metric used to compare different technologies in terms of maximum power output divided by the total mass of the device. The specific power of supercapacitors is 5 to 10 times greater than that of batteries. For example, while lithium-ion batteries have a specific power of 1-3 kW/kg, a typical supercapacitor has a specific power of about 10 kW/kg. This characteristic is particularly important in applications where bursts of energy need to be released quickly from the storage device.

Cycle life and safety

Supercapacitor batteries are safer than regular batteries when handled improperly. While batteries are known to explode due to overheating when short-circuited, supercapacitors do not heat up much due to their low internal resistance. Shorting a fully charged supercapacitor will result in a rapid release of stored energy, which can cause arcing and, potentially, damage to the device, but unlike batteries, the heat generated is not a problem.

Supercapacitors can be charged and discharged millions of times and have a virtually unlimited cycle life, whereas batteries only have a cycle life of 500 and more. This makes supercapacitors very useful in applications where energy needs to be stored and released frequently.

Disadvantages

Supercapacitors also have some disadvantages. One disadvantage is the low specific energy. Specific energy is a measure of the total energy stored in a device divided by its weight. While lithium-ion batteries commonly used in cell phones have a specific energy of 100-200 Wh/kg, supercapacitors typically store only 5 Wh/kg. This means that a supercapacitor with the same capacity (not capacitance) as a normal battery will weigh 40 times more. Do not confuse specific energy with specific power, which is a measure of the maximum output power of the device per weight.

Another disadvantage is the linear discharge voltage. For example, a battery rated at 2.7V will still output close to 2.7V when 50% charged, whereas a supercapacitor rated at 2.7V and 50% charged will output half of its maximum charge voltage - 1.35V. This means that the output voltage will be below the minimum operating voltage of a device running on a supercapacitor (such as a mobile phone), and the device must be turned off before all the charge in the capacitor can be used. The solution to this problem is to use a DC-DC converter. This approach introduces new difficulties, such as efficiency and power noise.

Cost is the third biggest disadvantage of currently available supercapacitors. The cost per Wh of supercapacitors is more than 20 times that of lithium-ion batteries. However, the cost can be reduced through new technologies and mass production of supercapacitor batteries.

Low specific energy, linear discharge voltage and high cost are the main reasons that prevent supercapacitors from replacing batteries in most applications.

Structure and performance of supercapacitors

The structure of supercapacitors is similar to that of electrolytic capacitors in that they consist of two foil electrodes, an electrolyte and a foil separator. The separator is sandwiched between the electrodes and the foil is rolled or folded into a shape that is usually cylindrical or rectangular. This folded form is placed into a casing, impregnated with electrolyte and sealed. The electrolyte used to construct the supercapacitor as well as the electrodes is different from the electrolyte used in ordinary electrolytic capacitors.

To store charge, supercapacitors use porous materials as separators so that ions can be stored in those pores at the atomic level. The most commonly used material in modern supercapacitors is activated carbon. The fact that carbon is not a good insulator leads to a maximum operating voltage limit of less than 3 V. Activated carbon is not a perfect material for another reason: the size of the charge carriers is comparable to the pores in the material, and some of them cannot fit into smaller pores, resulting in reduced storage capacity.

One of the most exciting materials in supercapacitor research is graphene. Graphene is a substance composed of pure carbon arranged in a flat sheet that is only one atom thick. It is very porous and acts as an ion "sponge". The energy density achievable using graphene in supercapacitors is comparable to that in batteries. However, although prototypes of graphene supercapacitors have been produced as proof of concept, graphene is difficult to produce in industrial quantities and expensive, which has delayed the use of this technology. Even so, graphene supercapacitors are the most promising candidate for future advances in supercapacitor technology.

Applications of supercapacitors

Since supercapacitors fill the gap between batteries and capacitors, they can be used in a wide variety of applications. One interesting application is energy storage in KERS, or dynamic braking systems (kinetic energy recovery systems) in the automotive industry. The main problem in such systems is to build energy storage devices that can quickly store large amounts of energy. One approach is to use a generator that converts kinetic energy into electrical energy and stores it in a supercapacitor. This energy can be reused later to provide acceleration power.

Another example is low-power applications where high capacity is not necessary, but a high life cycle or fast charging is important. Such applications are photographic flashes, MP3 players, static memory (SRAM), which requires a low-power constant voltage source to retain information, and many more.

Possible future applications for supercapacitors include mobile phones, laptops, electric cars, and all other devices that currently use batteries for power. The most exciting advantage from a practical point of view is that they charge very quickly, which means that plugging an electric car into a charger for a few minutes is enough to fully charge the battery.