Defense Intelligence Reference Document Ultracapacitors As Energy And Power Storage Devices

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Chapter 4: Recent Developments
There have been several recent advances in ultracapacitors focused on increasing
energy density by using different active materials24 and designs, 25 activated carbons,2 6
polymers, 27 and metal oxides. 28 Conducting polymers have good performance, 29 but
lack microfabrication protocols. Oxide-based thin-film ultracaps have shown high
performance, approaching the theoretical limit for capacitance (~1,000 F/g for MnO2). 30
However, the poor electrical conductivity and high impedances associated with surface
intercalation redox reactions of these oxides have limited practical film thicknesses to a
few microns. Carbon nanotubes have been added to the films to increase the electrical
conductivity, but the complexity of manufacturing limits practical applications of such
composite electrodes. In this section we will discuss advances in application of CNT and
advanced carbons to ultracapacitor materials, the application of thin-film manufacturing
processes, and a revolutionary new super capacitor using a giant magneto capacitive
effect.
CNTS AND ADVANCED CARBONS
Carbon electrodes constitute both electrodes in a symmetric ultracapacitor and one of
the electrodes in an asymmetric, or pseudocapacitive, design. Improvements to the
carbon electrode rely upon increasing the specific capacitance (in Farads per gram).
These improvements come by tailoring the surface area and porosity to achieve the
best balance that maximizes the interaction with the electrolyte. There is a linear
relationship to the surface area and the capacitance up to a point where capacitance
plateaus with activated carbons. By controlling the porosity and surface area, it is
possible to increase the capacitance beyond this plateau. Carbon nanotubes could
provide performance increases with aligned CNT forests of tailored sizes. The
characteristics of an ultracapacitor are highly dependent on the nanostructure of the
carbon used for the thin-film electrodes. Advanced carbons will provide better control
over the pore size and distribution, leading to an expected 50- to 100-percent
improvement over the carbons in use today.
Carbon nanotubes can be produced with a wide variety of properties. Depending on
synthesis parameters, nanotubes can be single walled or multiwalled, with varying
numbers of tubes. CNT diameters can be tailored from a few nanometers to tens of
nanometers, with lengths up to hundreds of microns. CNTs can be grown in random
orientations or as aligned forests. CNTs have a fully accessible surface area and very
high electrical conductivity. Methods for incorporating CNTs into electrodes for ultracaps
include using CNTs as an additive for conductivity enhancement, creating dense mats of
randomly oriented tubes, and creating electrodes from vertically aligned forests of
tubes. Initial results of CNT-enabled ultracaps tended to show much lower capacitance
than expected, which has been attributed to the hydrophobic nature of the CNT walls.
Surface functionalization is a common approach to mitigating the hydrophobicity issues
and thus enabling higher capacitance. Another benefit of the functionalization is the
ability to introduce and control pseudocapacitance.
Most efforts in CNT ultracaps are directed toward vertically aligned forests. It is possible
to controllably grow a dense, aligned forest that is perpendicular to the current collector.
The size and density of the tubes and the number of walls can be controlled with
catalyst design and reaction parameters. Manipulation of the CNT forest leads to
increased capacitance by fine-tuning the distance between tubes. Additionally, fine-
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