PhD Thesis - Cranfield University

Chapter 3
Positive
Terminal
Current
Collector
Seperator
Electrode Electrode
+
+
+
+
+
+
Electrolyte Electrolyte
-
-
-
-
-
-
Current
Collector
83
Negative
Terminal
Figure 3.15 Basic cell construction of an ultracapacitor
10um
source : Universität Würzburg
Ultracapacitors are not constrained to the same physical limitations as dielectric capacitors.
The discharge characteristics and equivalent circuits of ultracapacitors are similar to
conventional low farad capacitors but there are some fundamentally different properties
between the two types. The large capacitance ultracapacitors arise from the very large
specific area obtainable from the use of porous nano-carbon materials. Based on charging
and discharging the interfaces of high specific-area materials such as porous carbon materials
or porous oxides of some metals, these devices are able to store and releases immense
amounts of electric charge and corresponding energy at high densities (expressed in Wh/kg).
Hence, they can be operated at specific power densities (expressed in W/kg) higher than
batteries [103]. In addition, their capacitance for a given physical size of the device is much
higher to electrolytic capacitors. Comparing a 350F - 2.5V ultracapacitor (Maxwell BCAP
0350F, length = 62mm, diameter = 33mm, weight = 60g) with a 2200µF –100V electrolytic
capacitor ( Evox-Rifa PEH200, length = 60mm, diameter = 35, weight = 85g) shows that
the energy density of the ultracapacitor is approximately 140 times greater than the
electrolytic capacitor. For this reason proprietary terms such as Ultracapacitors and
Supercapacitors have been used to describe the high-energy storage capability of these devices.
A significant difference between charging and discharging an ultracapacitor compared to
charging and discharging a battery system is that there is always an intrinsic increase in