22-06-2013, 11:58 AM
FUNDAMENTAL OF ELECTROCHEMICAL CAPACITOR DESIGN AND OPERATION
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A simple EC canbe constructed by inserting two conductors
in a beaker containing an electrolyte, for examplg two carbon
rods in salt water (Fig. 1). Initially there is no measurable
voltage between the two rods, but when the switch is closed
and cunent is caused to flow from one rod to the other by a
battert charge separation is naturally created at each liqujdsolid
interface. This effectively creates two capacitors that
are series-connected by the electrolyte. Voltage persists after
the switch is opened-energy has been stored. In this state,
solvated ions in the electrolyte are attracted to the solid surface
by an equal but opposite charge in the solid. These two parallel
regions of charge form the source of the term "double layer."
Charge separation is measured in molecular dimensions (1.e.,
few angstroms), and the surface area is measured in thousands
of square meters per gram of electrode matedal, creating 5 kF
capacitors that can be hand-held.
Electric double-layer capacitor
Electric double-layer capacitors, also known as supercapacitors,
electrochemical double layer capacitors (EDLCs), or ultracapacitors, are
electrochemical capacitors that have an unusually high energy density when
compared to common capacitors, typically on the order of thousands of times
greater than a high capacity electrolytic capacitor. For instance, a typical D-cell
sized electrolytic capacitor will have a capacitance in the range of tens of
millifarads. The same size electric double-layer capacitor would have a
capacitance of several farads, an improvement of about two or three orders of
magnitude in capacitance, but usually at a lower working voltage. Larger,
commercial electric doubleJayer capacitors have capacities as high as 5,000
farads.[1] The highest energy density in production is 30 Whltg.lzJ
Concept
In a conventional capacitor, energy is stored by the
removal of charge carriers, typically electrons, from one
metal plate and depositing them on another. This charge
separation creates a potential between the two plates,
which can be harnessed in an external circuit. The total
energy stored in this fashion is proportional to both the
number of charges stored and the potential between the
plates. The number of charges stored is essentially a
function of size and the material properties of the plates,
while the potential between the plates is limited by the
dielectric breakdown. Different materials sandwiched
between the plates to separate them result in different
voltages to be stored. Optimizing the material leads to
higher energy densities for any given size of capacitor.
History
The electric double-layer capacitor effect was first noticed in 1957 by General Electric engineers experimenting
with devices using porous carbon electrode.[9] It was believed that the energy was stored in the carbon pores and it
exhibited "exceptionally high capacitance", although the mechanism was unknown at that time.
General Electric did not immediately follow up on this work, and the modern version of the devices were eventually
developed by researchers at Standard Oil of Ohio in 1966, after they accidentally re-discovered the effect while
working on experimental fuel cell designs.[10] Th"ir cell design used two layers of activated charcoal separated by a
thin porous insulator, and this basic mechanical design remains the basis of most electric double-layer capacitors to
this day.
Standard Oil also failed to commercialize their invention, licensing the technology to NEC, who finally marketed
the results as "supercapacitors" in 1978,to provide backup power for maintaining compute, rn"*ory.[10] The market
expanded slowly for a time, but starting around the mid-1990s various advances in materials science and simple
development of the existing systems led to rapidly improving performance and an equally rapid reduction in cost.