Introduction
Electrochemical capacitors
provide a mode of electrical charge- and energy-storage and
delivery, complementary to that by batteries.
The capacitance of a capacitor
is proportional to the area of the contact plates and the dielectric constant
of the medium between the plates, and it is inversely proportional to the
separation between the plates. In relation to electrochemical capacitors, to be
discussed below, the capacitance of dielectric capacitors is very small being
on the order of microfarads or nanofarads for small
devices on the order of mm or cm in dimensions.
Electrochemical capacitors are
a special kind of capacitor based on charging and discharging the interfaces
of high specific-area materials such as porous carbon materials or porous oxides of some metals. They can store electric charge and
corresponding energy at
high densities in an highly reversible way, as does a regular capacitor, and hence can
be operated at specific power
densities (in watts/kg) substantially higher than can most batteries. Their
capacitance for a given size of the device is thus much higher, by a factor of
10,000 or so, than those achievable with regular capacitors. For this reason
proprietary names such as "Supercapacitors"
or "Ultracapacitors" have been coined to
describe their performance.
While they function formally
like rechargeable batteries in storing or delivering electric charge, their
mechanisms of charge storage are quite different, in most cases, from those
operating in batteries. Thus, electrochemical capacitors are not substitutes
for batteries but rather are to be regarded as complementary to them for
charge storage or delivery. They can offer advantageously fast charging or
discharging rates over most batteries of comparable volume but their energy
density is usually less, by a factor of 3 to 4, than that of batteries. Their
high power or power densities, however, enables them to be employed in
interesting complementary ways in hybrid systems with batteries.
The double-layer capacitance at electrode interfaces
An
important class of electrochemical capacitors utilizes the so-called double-layer capacitance
that arises at all electrode
interfaces with electrolyte
solutions or ionic melts.
The specific capacitances of electrode double layers are very large, some
10,000 times those of ordinary dielectric capacitors, per cm2 area.
The reason for this is that the separation of charges in electrochemical
double layers is on the order of 0.3-0.5 nanometers (nm = one billionth of a
meter) instead of 10 to 100 nm with oxide-film dielectrics (electrolytic
capacitors) or 1000 nm with very thin mica or polystyrene
dielectric-film hardware capacitors.
Hence, it is seen that with
large specific-area porous electrodes, for example at carbons having say 1000 m2/g of material and exhibiting, say,
15 µF/(real
cm2) of double-layer capacitance in some suitable electrolyte
solution, the accessible capacitance "C" is 1000 (m2/g) ×
10,000 (cm2/m2) × 15 (µF/cm2) = 150 million
µF/g, that is 150 farads/g,
a very large capacitance! Hence the term "supercapacitors"
or "ultracapacitors" for devices based on
double-layer capacitance at high-area substrates.
The high degree of
reversibility of charge acceptance and delivery, and hence capability for
excellent operating power
levels compared with batteries
of comparable size arises because no slow chemical processes or phase changes
take place between charge
and discharge
as they do in most battery-type electrical charge generating devices.
It is the essence of
battery-type charge/discharge processes that faradaic processes take
place leading to major chemical and structural changes of the electrochemical
reactive materials, for example conversions of lead dioxide to lead sulfate and
lead metal to lead sulfate in discharge of the lead-acid battery which, as is
well known, limits charge/discharge to a cycle life of 1000 to
3000, depending on rates of charge and discharge, and temperature. By contrast,
electrochemical double-layer and oxide-type capacitors can exhibit cycle lives
up to one million under suitable conditions. This is because only storage and
delivery of electrostatic charge takes place at the extended two-dimensional
interface of high-area materials and no irreversible or slow chemical phase
changes take place as they do between three-dimensional chemical materials in
rechargeable batteries. This is a fundamental difference between the
electrochemical behavior and properties of electrochemical capacitors relative
to those of batteries.
It was stated earlier in this article that charging and discharging of
electrochemical capacitors has commonly been perceived as a process much more
reversible than that for batteries and hence being capable of operation at high
power densities. While, in
practice, this is largely true, charging of the high-area, porous-electrode
structures that are required for achieving large capacitance densities
(farads/g or farads/cm3) encounters limitations of rate due to the
distributed electrolytic and contact resistances within the
pore structure of such materials.
In the simplest analysis, any
practical capacitor device behaves as if an ohmic
resistance is in series
with it, the so-called equivalent series resistance. The presence of real or
equivalent series resistance in the operating equivalent circuit of any
capacitor introduces an ir potential
drop in the process of charging or discharging and this drop
depends, of course, on the charging rate (current) leading to
distortion of the charging curve of accumulated charge against voltage, in time. When the
distributed resistance effect also operates, as it normally does, the
distortion effect becomes more complex but has been experimentally and
computationally evaluated.
This situation leads to
limitation of the rates of which charging or discharging of porous
capacitor electrodes can be conducted and is additional to any equivalent series
resistance effects a practical cell may experience due to cell design and electrolyte resistance.
It must be stated, however,
that in state-of-the-art developments of electrochemical capacitors, using aqueous-solution
electrolytes, the above distributed-resistance effect has been substantially
minimized so that devices having high operating power have been successfully
engineered and marketed. Nevertheless, with non-aqueous electrolyte capacitors,
which have higher operating voltages up to 3.0 to 3.5 V (hence 9 to 12 times
energy density which depends on the square of maximum operating
voltage), the distributed ohmic effects are more
significant so that achievable operating power levels are less than those
attainable with aqueous electrolyte devices.
Electrochemical capacitors based on pseudocapacitance
A different
kind of capacitance
can arise at electrodes
of certain kinds, for example ruthenium dioxide, when the extent of faradaically admitted charge depends linearly,
or approximately linearly, on the applied voltage. For such a
situation, the electrode behavior is equivalent to, and measurable as, a
capacitance. This capacitance can be large but it is faradaic
and not electrostatic in origin. This is hence an important difference from the
nature of double-layer
capacitance, so it is called "pseudocapacitance".
This kind of pseudocapacitance can originate when an
electrochemical charge-transfer
process takes place to an extent limited by a finite quantity of
reagent or of available surface. Several examples of pseudocapacitance
can arise, but the capacitance function is usually not constant and, in fact,
is usually appreciably dependent on potential or state of charge.
However, when the process is
surface limited, and is proceeding in several one-electron stages, a broad
range of significant capacitance values arises as is found with ruthenium
dioxide electrodes where the pseudocapacitance is
almost constant (within 5%) over the full operating voltage range. Some other
metal oxides behave similarly but only over smaller operating voltage ranges.
The ruthenium dioxide pseudocapacitance provides one
of the best examples of electrochemical (pseudo)capacitance as, in addition to the
almost constant capacitance over a wide voltage range (over a 1.4 V range), its
reversibility is excellent, with a cycle life over several
hundred-thousand cycles. Furthermore, the pseudocapacitance
can increase the capacitance of an electrochemical capacitor by as much as an
order of magnitude over that of the double-layer capacitance. However, its cost
prevents its large-scale use so that it has been employed mainly in military
applications.
General
industrial production of electrochemical capacitors follows that of battery
cell procedures with automatic production-line machinery. Cell designs are of
various kinds including cylindrical, prismatic, button, or coin
types, with some larger embodiments being of cake-tin sizes or larger, and some
multi-cell series for higher voltage with bipolar electrodes having
edge seals. In series
configurations for high-voltage applications, balancing of unit cell
performance and behavior is a technological challenge.
Another application is in electrical research experiments where very high energy and high-rate
discharges are required, for example through gases for high-energy spark or arc
generation; cold-start assist for diesel locomotives; emergency back-up power
for computer systems; stationary power-system load leveling or bridging
for short-period power outages; energy source for initial heating of catalytic
converter units; energy collection and storage from windmill
dynamos.
* Abstracted
from Electrochemistry
Encyclopedia
(http://electrochem.cwru.edu/ed/encycl/)
ELECTROCHEMICAL
CAPACITORS
Their Nature, Function, and Applications
By Brian E. Conway