Supercapacitor (EDLC)

A capacitor that stores charge across a gap thinner than a single ion — no chemical reaction, just electrostatic attraction, repeated a million times without wearing out.

An electric double-layer capacitor stores electrical energy by attracting ions from a liquid electrolyte onto the surface of two porous carbon electrodes. No chemical bonds are broken or formed — the ions simply pile up on the electrode surface when voltage is applied and disperse back into solution when it is removed. This purely physical process is reversible within milliseconds, meaning the device charges and discharges in fractions of a second rather than minutes or hours. The trade-off is modest energy density — roughly ten times less than a lithium-ion cell by weight — but what the EDLC lacks in energy it more than makes up in power delivery, cycle life, and operating temperature range.

Each electrode is made from activated carbon — a highly porous form of carbon with a surface area typically between 1,000 and 3,000 m² per gram, roughly the footprint of several football pitches packed into a teaspoon. When a voltage is applied (typically up to 2.7 V per cell for organic electrolyte systems), positive ions from the electrolyte crowd against the negative electrode and negative ions crowd against the positive electrode, forming two molecular-scale 'double layers' — hence the name. The thickness of this double layer is on the order of a nanometre, which makes the effective capacitance per unit area enormous: a conventional film capacitor has electrode gaps of micrometres; an EDLC's gap is a thousand times thinner. Series-connected cells build voltage to the levels needed for industrial applications. Because there is no chemical reaction, the device neither degrades from cycling nor requires the minutes needed to drive ions into a crystal lattice: full charge and discharge happen in seconds.

In plain terms

Think of a battery as a water tank that fills up slowly and drains slowly. A supercapacitor is a tiny sponge instead — you can squeeze it to get water out instantly or soak it full in two seconds, and you can do that a million times before it wears out. It just doesn't hold as much water as the tank.

Also called: ultracapacitor, electric double-layer capacitor, electrochemical capacitor, EDLC.

Why it matters

Cycle life of 500,000 to 1,000,000 cycles dwarfs any rechargeable battery, making EDLCs suitable for applications that cycle dozens of times per day for decades.
Power density of roughly 10–25 kW/kg (cell level) enables recovery of regenerative braking energy in seconds — an energy window too short for batteries to absorb efficiently.
Operable from −40 °C to +65 °C without significant performance loss, covering environments hostile to lithium chemistries (cold climates, under-hood automotive).
No chemical reactions means no thermal runaway, no gassing, and no 'memory effect' — a cell can sit partially charged indefinitely without degradation from state-of-charge history.

Where it lives

Maxwell BCAP3000 (3,000 F at 2.7 V, 475 g — the workhorse cell for heavy-transport hybrid buses and industrial UPS)
Skeleton Technologies SkelCap SCX series (curved graphene electrode, up to 5,000 F per cell)
Regenerative braking systems on hybrid diesel-electric city buses (Volvo, MAN, and others) — captures and re-delivers braking energy at bus stops
Lamborghini Sián FKP 37 (first production car to use supercapacitors as the sole hybrid energy buffer; 48 V system, regenerative braking below 130 km/h)
Wind turbine blade pitch control (replacing lead-acid backup batteries that failed in cold conditions)
Port cranes at container terminals (captures deceleration energy from lowering containers)

Key numbers

Cell voltage (organic electrolyte): 2.7–3.0 V (Per cell; Maxwell BCAP3000 rated 2.7 V, P300 variant rated 3.0 V)
Capacitance (Maxwell BCAP3000): 3,000 F (At 2.7 V, 475 g cell; from Maxwell datasheet 3003279)
Projected cycle life: 500,000–1,000,000 cycles (Constant-current charge-discharge to ½ rated voltage; per Maxwell and DOE data)
Gravimetric energy density: 4–10 Wh/kg (Cell level; ~6.9 Wh/kg for BCAP3000 per Maxwell datasheet; 10–100× less than Li-ion)
Usable specific power: ~10–25 kW/kg (Cell level; 12.3 kW/kg typical for BCAP3000 per Maxwell datasheet)
Round-trip efficiency: ~92 % (At short discharge durations; per DOE Technology Strategy Assessment (2023))
Operating temperature range: −40 °C to +65 °C (Per Maxwell and EASE datasheets; batteries typically limited to −20 °C)
Electrode surface area: 1,000–3,000 m²/g (Activated carbon BET surface area; higher end via KOH activation)

Surprising facts

The Maxwell BCAP3000 — a cylinder roughly the size of a large soda can — holds 3,000 farads of capacitance. A typical film capacitor found in consumer electronics holds about 0.000001 farads: the EDLC packs three billion times more capacitance into a similar volume.
The electric double layer that stores the charge is about one nanometre thick — roughly ten times the diameter of a hydrogen atom — yet it must withstand 2.7 V without the electrolyte decomposing, making the effective electric field across it comparable to lightning inside a thundercloud.
A single BCAP3000 cell rated for 1,000,000 cycles, cycled 40 times per day, would outlast 68 years of continuous operation — longer than the expected service life of most buildings that might house it.
Activated carbon used in EDLC electrodes is typically made from coconut shells. A single gram of the material, unfolded, would cover an area the size of a squash court.

Open questions

Anomalous capacitance in sub-nanometre pores: Classical electrostatic theory predicts that pores smaller than the solvated ion diameter cannot contribute to capacitance — the ions simply cannot enter. Yet experiments in the early 2000s found that carbons with pores in the 0.7–0.8 nm range, smaller than the solvated electrolyte ions, showed anomalously high capacitance. Molecular dynamics simulations later suggested that ions shed part of their solvation shell to squeeze in, enhancing charge density in a way the original double-layer model did not predict. The precise mechanism — which solvation shell configurations are energetically accessible in these extreme geometries — is still an active area of research.

A short history

The first device deliberately exploiting double-layer charge storage was patented on 23 July 1957 by H.I. Becker of General Electric (U.S. Patent 2,800,616). It was an experimental curiosity — small, leaky, and of unclear use. Standard Oil of Ohio (SOHIO) filed a more practical patent in 1966, and NEC in Japan commercialised small 'supercaps' for clock backup memory in the early 1970s under the 'Supercapacitor' trademark. Through the 1980s and 1990s, interest in the devices grew in parallel with the discovery of higher-surface-area carbon materials. Maxwell Technologies (San Diego) became the dominant producer of large cylindrical cells for transportation and industrial applications. The 2000s brought the realisation that hybrid bus and tram regenerative braking was an ideal match for EDLC cycle life and power density — systems that would have destroyed battery packs in two years began running on the same EDLC modules for over a decade. Skeleton Technologies, founded in Estonia in 2009, introduced curved-graphene electrodes in the 2010s, boosting energy density by up to 72 % over conventional activated carbon. The Maxwell product line was acquired by Tesla in 2019.

Sources

Maxwell Technologies — BCAP3000 P270 K04/K05 Datasheet (document 3003279-EN.2, 2021) (https://maxwell.com/wp-content/uploads/2021/09/3003279.2_Final-DS_New-2.7V-3000F-Cell_20210406.pdf)
U.S. Department of Energy — Technology Strategy Assessment: Supercapacitors (2023) (https://www.energy.gov/sites/default/files/2023-07/Technology%20Strategy%20Assessment%20-%20Supercapacitors.pdf)
EASE (European Association for Storage of Energy) — EDLC Technology Description, cycle life and efficiency data (https://energystorageeurope.eu/wp-content/uploads/2016/03/EASE_TD_EDLC.pdf)
Skeleton Technologies — What is Curved Graphene? (electrode energy density improvement) (https://www.skeletontech.com/skeleton-blog/what-is-curved-graphene)