Supercapacitor

Also called electrochemical capacitor

Everyday term

In plain words

A device that stores charge on the surface of its electrodes instead of in a chemical reaction deep inside them. It holds less energy than a but charges in seconds and survives far more cycles, which suits materials whose surface is nearly all of them.

Going deeper

A supercapacitor stores charge at the surfaces of its electrodes rather than in a bulk chemical reaction. Nothing has to diffuse deep into the material, so it charges in seconds and lasts for very many cycles – at the cost of storing far less energy than a battery.

Two ways to store charge at a surface

The first is the electrical double layer: ions in the electrolyte gather against a charged electrode, with no reaction taking place. The stored charge is proportional to surface area and to voltage, and the process is as fast as ions can move, so it survives hundreds of thousands of cycles.

The second is pseudocapacitance: fast, reversible redox reactions confined to the surface, in which ions are adsorbed or a surface group changes oxidation state. This stores considerably more charge per area than double-layer charging while keeping much of its speed, because no phase has to grow and nothing has to diffuse through the bulk. The line between pseudocapacitance and a fast battery reaction is blurred, and the distinction is usually made by how the current scales with sweep rate.

Why 2D materials suit the job

If charge lives on surfaces, a material that is nearly all surface is the natural electrode. Activated carbons provide enormous area cheaply but conduct moderately; graphene-derived electrodes offer area with better conductivity, provided the sheets do not stack back together.

brought a different combination: conductivity along the sheets, redox-active surface groups, and spacing between sheets that ions can enter. Clay-like Ti₃C₂T_x films reached volumetric capacitances of order 900 F per cm³, well above carbon electrodes, and can be rolled or printed into thin, flexible electrodes. The gain is in capacitance per volume rather than per mass, which suits small devices more than vehicles.

The limits

Energy density is the fundamental one. Stored energy scales with capacitance and with the square of voltage, and aqueous electrolytes limit the voltage to around one volt; organic and electrolytes allow more but conduct worse. Supercapacitors therefore store far less energy per kilogram than batteries and complement them rather than replace them – in regenerative braking, power smoothing and backup.

Reported numbers deserve care: capacitance per gram of active material on a thin electrode flatters, while device-level energy and power including electrolyte, current collectors and packaging are what matter. Restacking of sheets, electrolyte access into dense films and self-discharge are the practical obstacles, and volumetric and gravimetric figures often point in opposite directions.

For specialists

Electrochemical storage by double-layer charging plus fast, surface-confined redox (pseudocapacitance). 2D electrodes suit it because stored charge follows accessible surface rather than bulk diffusion: MXene films combine metallic conductivity with redox-active surface groups and reach volumetric capacitances of order 900 F cm⁻³. The trade-offs are modest energy density and the need to stop the sheets restacking.

Where this comes from

Materials for electrochemical capacitors Simon and Gogotsi · Nature Materials 7, 845 (2008) cited by 16,449
Conductive two-dimensional titanium carbide ‘clay’ with high volumetric capacitance Ghidiu et al. · Nature 516, 78 (2014) cited by 6,028

Battery electrode

A battery stores energy by moving ions – in phones and cars, lithium ions – between two electrodes, the anode and the cathode. Charging pushes the ions into the anode, where they slip between the layers of a material such as graphite; using the battery lets them flow back. Layered and 2D materials matter because ions can enter and leave the gaps between layers, and because thin conductive flakes can help electrons reach every grain of an electrode.

A lithium- or sodium-ion cell shuttles ions through an electrolyte between a negative electrode (the anode on discharge) and a positive one (the cathode), while electrons pass through the external circuit. Graphite, the standard anode, intercalates lithium between its layers up to LiC₆, about 372 mAh/g; layered oxides such as LiCoO₂ and NMC dominate cathodes. 2D materials enter as conductive additives (graphene flakes replacing part of the carbon black), as higher-capacity anodes that alloy or convert (phosphorus, antimony, SnS₂, SnO), as hosts for larger ions such as sodium, which graphite takes up poorly, and as MXene electrodes. Their weaknesses are large volume change, irreversible first-cycle loss from the high surface area, low packing density and cost, so gains at cell level are usually far smaller than those claimed for the material.

Full explanation

Metal, semimetal and insulator

The three basic ways a solid treats electricity, set by how its electrons fill the energies open to them. In a metal they fill a band of energies only partly, so the smallest push sets them moving, and it conducts at any temperature. In an insulator they fill a band completely and a wide gap separates it from the next, so nothing can move. A semimetal sits in between: two bands just touch or overlap a little, leaving a small number of mobile electrons and holes. NbSe₂ is a metal, graphene a semimetal, hBN an insulator.

A classification by where the Fermi level falls in the band structure. In a metal it crosses a partly filled band, leaving a Fermi surface, a carrier density of order one per atom (about 10²² cm⁻³) and a resistivity that falls on cooling. In a band insulator it lies in a gap of several electronvolts; a semiconductor is an insulator whose gap is small enough to be doped or gated. A semimetal has a slight overlap between conduction and valence bands (graphite, bismuth, bulk WTe₂) or bands that touch at isolated points (graphene), giving low and nearly equal densities of electrons and holes. Interactions can open gaps that band theory does not predict: Mott insulators from half-filled bands, excitonic insulators from semimetals.

Full explanation

Gating

Changing a material by putting a voltage on a nearby electrode – a gate – separated from it by an insulator. The voltage pulls electrons into the material or pushes them out, as in every transistor. Because a 2D material is so thin, the gate reaches all of it, so the number of electrons can be dialled up and down continuously: a single device can be turned from insulator to metal, its magnetism strengthened, or superconductivity switched on. Two gates, above and below, can also apply an electric field across the layer.

Electrostatic control of carrier density and electric field through a gate coupled capacitively across a dielectric: the induced density equals the gate capacitance times the voltage beyond threshold, divided by the electron charge, reaching a few 10¹³ cm⁻² with oxide or hBN gates before breakdown. Dual gating sets density and perpendicular displacement field independently, opening the gap of bilayer graphene and tuning moiré flat bands. Ionic-liquid and solid-electrolyte gating form an electric double layer about a nanometre thick and reach 10¹⁴–10¹⁵ cm⁻², enough to induce superconductivity in MoS₂ or raise the Curie temperature of Fe₃GeTe₂, but can intercalate or react electrochemically and work only while the ions are mobile. Hysteresis from traps, contact effects and quantum capacitance are the usual caveats.

Full explanation