Environmental Engineering Reference
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for high power sources, such as power trains and electric vehicles. Extensive re-
search works are still carried out in order to obtain better performance. Asym-
metric EDLCs using different activated carbons in two electrodes, negative and
positive electrodes, were constructed and their performance was studied in non-
aqueous electrolyte. The capacitance and rate performance of these asymmetric
EDLCs were found to be governed predominantly by the pore structure of the
carbon in the negative electrode [108].
1.1.11.4.1
SUPER-CAPACITORS BASED ON ACTIVATED CARBON
MATERIAL
More than 20 years ago an experimental super-capacitor cell by using commercial
activated carbon fiber (ACF) cloth for each of the two electrodes and glass fiber
filter paper as separator in organic electrolyte was realized. At that time 36.2Wh/
kg specific energy, 11.1 kW/kg specific power and 36.5 F/g specific capacitance
were estimated. The specific capacitance was considered per gram of ACF. For
the ACF cloth made from phenolic resin, a specific surface area of 1500-2500
m
2
/g was estimated. The specific energy and specific power reached in practice at
this time are much lower than the above estimated values because these take into
consideration the overall weight of capacitor cell including its package. Activated
carbon composite electrodes for electrochemical capacitors have been also inves-
tigated. Thus for hydrous ruthenium oxide/activated carbon electrode in H
2
SO
4
electrolyte, an increase of specific capacitance from 243 F/g (for pure activated
carbon electrode) to 350 F/g for composite electrode where 35% is ruthenium
oxide is reported by scientists.
In other work for only 3.2% ruthenium oxide in the composite electrode, an
increase in the capacitance of 25% to a value of 324 F/g is reported. Other ex-
periments with ruthenium oxide/activated carbon composites used at positive
electrodes in electrochemical capacitors indicated increase of the specific capaci-
tance. Nickel hydroxide/activated carbon composite electrodes used in electro-
chemical capacitors provide significant increase of specific capacitance from 255
to 314 F/g. If manganese oxide/activated carbon composite electrodes are used
increase of specific capacitance takes place. Other materials used for activated
carbon composite electrodes have been found to increase the specific capacitance.
In hybrid or asymmetric electrochemical capacitors, one electrode is based on
activated carbon material and another one is based on another material (nickel hy-
droxide, manganese oxide, etc.). Higher specific capacitance or specific energy is
possible than in the case of symmetric capacitors, based only on activated carbon
electrodes [34, 45, 64].
It is usually anticipated that the capacitance of a porous carbon (expressed in
F g
−1
) will be proportional to its available surface area (in m
2
g
−1
). Whilst this re-
lationship is sometimes observed, in practice it usually represents an oversimpli-
fication. The major factors that contribute to what is often a complex (nonlinear)
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