Supercapacitors

Supercapacitors are electrical storage devices consisting of two electrodes, usually identical, separated by an electrolyte. Like batteries, they are rechargeable systems, also known as secondary generators, unlike primary cells or generators, which are not rechargeable, and fuel cells, which convert chemical energy directly into electrical energy.

The general operating principle of supercapacitors is based on the formation of an electrochemical double layer at the interface of an electrolyte and a polarizable electrode with a large specific surface area. Applying a potential difference across the entire device results in the electrostatic storage of charge at the two electrode-electrolyte interfaces, which behave like two capacitors in series. The system restores the accumulated charge with excellent efficiency and over a very large number of cycles. The electrical response of supercapacitors is similar to that of capacitors: they are capable of delivering high current densities for relatively short periods of time (high power); for an equivalent volume, they store 20 to 50 times less electrical energy than accumulators, but around 100 times more than conventional dielectric capacitors, hence their name "supercapacitors" and their vocation as secondary generators.

Small supercapacitors, delivering microampere to milliampere currents at just a few volts, have been on the market since the late 1970s. Their major application, which is still relevant today, is the backup of memories and other functions that need to be preserved during an interruption in the main power supply. As their cost and maintenance are generally lower than those of accumulators, and their lifespan much longer, they have been introduced in a number of portable equipment and backup power supplies. Since the early 1990s, however, the field has been revitalized by the emergence of larger supercapacitors for applications requiring greater energy and, above all, power. Their development is motivated by specific applications in which the energy-power trade-off is better satisfied than by accumulators or capacitors alone. In fact, supercapacitors are often used as auxiliaries to capacitors, accumulators or even fuel cells in hybrid systems, where each device plays a complementary role. This is the case with electric vehicles, where traction is provided by accumulators, while starting, acceleration and other occasional operations benefit from the power provided by supercapacitors, which can be recharged during braking. Generally speaking, combining a supercapacitor with an accumulator should enable the former to play a role in filtering power demands, thereby increasing the energy potential of the latter.

In the longer term, supercapacitors seem capable of achieving levels of...