What are supercapacitors?
Supercapacitors are electronic devices which are used to store extremely large amounts of electrical charge. They are also known as double-layer capacitors or ultracapacitors. Instead of using a conventional dielectric, 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 normal capacitors with the workings of an ordinary battery. Capacitances achieved using this technology can be as high as 12000 F. In comparison, the self-capacitance of the entire planet Earth is only about 710 µF, more than 15 million times less than the capacitance of a supercapacitor. While an ordinary electrostatic capacitor may have a high maximum operating voltage, the typical maximum charge voltage of a supercapacitor lies between 2.5 and 2.7 volts. Supercapacitors are polar devices, meaning they have to be connected to the circuit the right way, just like electrolyte capacitors. The electrical properties of these devices, especially their fast charge and discharge times, are very interesting for some applications, where supercapacitors may completely replace batteries.
A supercapacitor is a specially designed capacitor which has a very large capacitance. Supercapacitors combine the properties of capacitors and batteries into one device.
Supercapacitors have charge and discharge times comparable to those of ordinary capacitors. It is possible to achieve high charge and discharge currents due to their low internal resistance. Batteries usually take up to several hours to reach a fully charged state – a good example is a cell phone battery, while supercapacitors can be brought to the same charge state in less than two minutes.
The specific power of a battery or supercapacitor is a measure used to compare different technologies in terms of maximum power output divided by total mass of the device. Supercapacitors have a specific power 5 to 10 times greater than that of batteries. For example, while Li-ion batteries have a specific power of 1 – 3 kW/kg, the specific power of a typical supercapacitor is around 10 kW/kg. This property is especially important in applications that require quick bursts of energy to be released from the storage device.
Cycle life and safety
Supercapacitor batteries are safer than ordinary batteries when mistreated. While batteries are known to explode due to excessive heating when short circuited, supercapacitors do not heat as much due to their low internal resistance. Shorting a fully charged supercapacitor will cause a quick release of the stored energy which can cause electrical arcing, and might cause damage to the device, but unlike batteries, the generated heat is not a concern.
Supercapacitors can be charged and discharged millions of times and have a virtually unlimited cycle life, while batteries only have a cycle life of 500 times and higher. This makes supercapacitors very useful in applications where frequent storage and release of energy is required.
Supercapacitors come with some disadvantages as well. One disadvantage is a relatively low specific energy. The specific energy is a measure of total amount of energy stored in the device divided by its weight. While Li-ion batteries commonly used in cell phones have a specific energy of 100-200 Wh/kg, supercapacitors may only store typically 5 Wh/kg. This means that a supercapacitor that has the same capacity (not capacitance) as a regular battery would weigh up to 40 times as much. The specific energy is not to be confused with the specific power, which is a measure of maximum output power of a device per weight.
Another disadvantage is a linear discharge voltage. For example, a battery rated at 2.7V, when at 50% charge would still output a voltage close to 2.7V, while a supercapacitor rated at 2.7V at 50% charge would output exactly half of its maximum charge voltage – 1.35V. This means that the output voltage would fall below the minimal operating voltage of the device running on a supercapacitor, for example a cellphone, and the device would have to shut down before using all the charge in the capacitor. A solution to this problem is using DC-DC converters. This approach introduces new difficulties, such as efficiency and power noise.
Cost is the third major disadvantage of currently available supercapacitors. The cost per Wh of a supercapacitor is more than 20 times higher than that of Li-ion batteries. However, 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 preventing supercapacitors from replacing batteries in most applications.
Construction and properties of supercapacitors
The construction of supercapacitor is similar to the construction 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, usually cylindrical or rectangular. This folded form is placed into a housing, impregnated with electrolyte and hermetically sealed. The electrolyte used in the construction of supercapacitors as well as the electrodes, are different from those used in ordinary electrolytic capacitors.
In order to store electrical charge, a supercapacitor uses porous materials as separators in order to store ions in those pores at an atomic level. The most commonly used material in modern supercapacitors is activated charcoal. The fact that carbon is not a good insulator results in a maximum operating voltage limited to under 3 V. Activated charcoal is not the perfect material for another reason: the charge carriers are comparable in size to the pores in the material and some of them cannot fit into the smaller pores, resulting in a reduced storage capacity.
One of the most exciting materials used in supercapacitor research is graphene. Graphene is a substance consisted of pure carbon, arranged in a planar sheet only one atom thick. It is extremely porous, acting as an ion “sponge”. Energy densities achievable using graphene in supercapacitors are comparable to energy densities found in batteries. However, even though prototypes of graphene supercapacitors have been made as a proof of concept, graphene is difficult and expensive to produce in industrial quantities, which postpones the use of this technology. Even so, graphene supercapacitors are the most promising candidate for future supercapacitor technology advances.
Applications for supercapacitors
Since supercapacitors bridge the gap between batteries and capacitors, they may be used in a wide variety of applications. One interesting application is the storage of energy in KERS, or dynamic braking systems (Kinetic Energy Recovery System) in automotive industry. The main problem in such systems is building an energy storage device capable of rapidly storing large amounts of energy. One approach is to use an electrical generator which will convert kinetic energy to electrical energy and store it in a supercapacitor. This energy can later be reused to provide power for acceleration.
Another example are low-power applications where a high capacity is not imperative, but a high life cycle or quick recharging is important. Such applications are photographic flash, MP3 players, static memories (SRAM) which need a low power constant voltage source to retain information and so on.
Possible future supercapacitor applications are in cell phones, laptops, electric cars and all other devices that currently run on batteries. The most exciting advantage from a practical perspective is their very fast recharge rate, which would mean that plugging an electric car into a charger for a few minutes would be enough to fully charge the battery.