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Tuesday, 1 February 2011

Double-layers and supercapacitors

A double layer (DL), or electrical double layer, is a charge distribution that appears on the surface of an object when it is placed into a liquid. The object might be a solid particle, a gas bubble or a liquid droplet. The double layer is created by two parallel layers of charge, surrounding the object. The first layer is attached to the surface, with a charge either positive or negative. This layer is built by the ions adsorbed* onto the object due to chemical interactions. The second layer is composed of ions attracted to the surface charge via the coulomb force, electrically screening the first layer. This second layer is loosely associated with the object: it is made of free ions which move in the fluid under the influence of electric attraction and thermal motion rather than being firmly anchored. It is thus called the diffuse layer.
DL plays a fundamental role in many real-world systems. For instance, milk exists only because fat droplets are covered with a double layer that prevent their coagulation into butter. DLs exist in practically all heterogeneous fluid-based systems, such as blood, paints, inks, ceramic slurries and cement slurries.

* In general, adsorption is the adhesion of atoms, ions, biomolecules or molecules of gas, liquid, or dissolved solids to a surface. This process creates a film of the adsorbate (the molecules or atoms being accumulated) on the surface of the adsorbent.More on double-layers at http://en.wikipedia.org/wiki/Double_layer_(interfacial)
http://depts.washington.edu/solgel/pages/courses/MSE_502/Electrostatic_Stabilization.html



An electric double-layer capacitor, also known as supercapacitor, supercondenser, pseudocapacitor, electrochemical double layer capacitor(EDLC), or ultracapacitor, is an electrochemical capacitor with relatively highenergy density, typically on the order of thousands of times greater than anelectrolytic capacitor.
Supercapacitors are able to store about a million times more charges than conventional capacitors. The principle is that of the "double layer". The charge is stored in the so called Helmholtz double layer. When voltage is applied, ions of opposite charge accumulate at the electrodes and the double layer is formed. Its thickness is in the range of several angstroms. Usually, highly porous carbon is used as electrode material, to create a fractal surface. Due to this very high specific electrode surface and the very small distances within the double layer, capacities of several hundreds farads can be achieved. As charge storage is only physical and not chemical, as in the case of batteries.

Some researchers in the US have made the first high-frequency AC supercapacitors containing graphene electrodes. The devices, which are much smaller than conventional capacitors, could be used in applications like computer processing units and other tiny integrated circuits.