AAWSAP DIRD, Ultracapacitors as Energy and Power Storage Devices, November 2010

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of some CNT types. Carbon nanotubes have been a rich area of study for many
applications, which has translated to a broad understanding of synthesis techniques to
produce CNTs with tailorable properties.
Graphene can be thought of as a sheet of graphite, either a single layer or a few layers
thick. The two-dimensional nature of graphene leads to extremely high electron
transport across the surface, interesting magnetic properties, high strength, and a large
accessible surface area. 18 Graphene is generally made by exfoliation/separation of
graphite or grown by CVD processes. Derivations from both of these methods can
produce single-layer graphene or few-layer graphene. These methods include, but are
not limited to, CVD methods, epitaxial growth, solvothermal synthesis, micromechanical
exfoliation, and colloidal synthesis. 19 Graphene has generated an enormous amount of
attention due to its potential for transforming electronics. A number of approaches to
graphene synthesis are being used that consider purity, scalability, and cost.
Carbide-derived carbons are a unique class of porous carbons with extremely fine
control over pore size. 20 These porous carbons are derived by chlorination of metal
carbides at very high temperatures. At temperatures of 500-1000°C, metals and
metalloids are removed as chlorides, leaving behind a finely tunable nanoporous carbon.
These porous carbons have a pore-size distribution that is tunable within
0.05 nm. The extremely accurate tunability has allowed these systems to serve as a
model for the relationship of pore size and capacitance, thereby allowing for design of
materials to be tailored for maximum capacitance within a specific system. The local
maximum in capacitance is dependent on the ion salvation sphere, which is a product of
the electrolyte ion, solvent, and interaction with the pores of the electrode. These
findings will lead to new electrode materials and designs.
Activated carbon and other porous carbon derivatives are commonly used for
symmetric ultracapacitors. Metal oxides such as ruthenium oxide and conductive
polymers replace one of the electrodes to create an asymmetric system. Asymmetric
electrodes greatly increase the storage capacity of ultracapacitors. These are a different
class of capacitor, as they store their charge both in the electrical double layer and
using a surface redox (faradaic) reaction (in the same manner as a battery). While
these undergo electron transfer reactions, they behave in a capacitive fashion. Often
termed pseudocapacitor, this class of materials includes a variety of metal oxides and
conductive polymers. The capacitance of an asymmetric electrode is twice that of a
symmetric design. The electron transfer electrode essentially has a fixed potential,
whereas the potential of the carbon electrode changes with state of charge. Additional
storage capacity arises from the higher working voltage, due to the different rest
potentials of the different electrodes.
Asymmetric electrodes are becoming increasingly common due to the increased energy­
storage capability. As energy is proportional to the voltage squared, the higher
operating voltage has profound effects on the capacity. Ruthenium oxide (RuO2) has
been shown to give capacitance of as high as 1300 F/cm 3• By comparison,
carbonaceous materials in an aqueous electrolyte have a capacitance of 110-125 F/cm3.
There are many additional metal oxides being investigated for insertion into the
ultracapacitor system. Many of these have their roots in lithium ion battery cathode
materials. As ultracaps gain wider acceptance, market size is increasing rapidly and is
expected to grow at an increasing pace. Investigations are being conducted into
alternative metal oxide systems to ensure that cost, supply, and performance can be
met on a global scale. Manganese oxide (MnO2) and nickel metal oxides/hydroxides are
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