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

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there is room for improvement on these values. Optimization of graphene synthesis to
produce finer control is an area receiving increased attention in research and
development activities in universities, national labs, and industrial research facilities.
Lockheed Martin is actively engaged in research investigating graphene and applications
where graphene could be a critical differentiator. There are a number of companies that
supply graphene made from various methods, as well as emerging startups geared
toward producing graphene-based ultracapacitors or materials designed for ultracaps.
Figure 8. Transmission Electron Microscope Image of Graphene. This could be an excellent ultracapacitor
electrode material with its high surface area and excellent conductivity. 33
Manipulation of the porosity of high surface area carbons leads to the largest difference
in specific capacitance. Understanding the relationship between the electrolyte ion size
and the carbon pore size is critical to improving performance. There are a number of
strategies being investigated for fine control over the pore-size distribution to increase
the specific capacitance. The most common methods used currently are template
methods and carbide-derived carbons. Template methods create a controlled
mesoporous structure with a fairly narrow range. These structures have pores that
range from 2 to 10 nanometers and are maximized to pore sizes roughly twice that of
the solvated ions. The template process involves filling the pores of an inorganic
template host with a carbon precursor (such as an alumina template). The template is
removed after carbonization by acid treatment. The pore size is then dictated by the
template as the pores are the remaining void space once occupied by the template.
Similar methods have shown that smaller pores, including those less than two
nanometers, may provide high specific capacitance. The realization that smaller pores
contribute to charge storage in an electric double layer has led to the need to develop a
better understanding of the charge storage mechanism.
Carbide-derived carbons have a unique pore-size distribution that is tunable with sub­
angstrom accuracy. These have served as models to study the charge storage behavior
and ion adsorption in pore sizes ranging from 0.6 nm to 1.1 nm. 34 The normalized
capacitance decreases with decreasing pore size until a critical value is reached.
Figure 9 shows the relationship between average pore size and the normalized specific
capacitance. Pore sizes smaller than one nanometer significantly contribute to the
charge storage despite the fact that the solvated ion size is larger than the pore
diameter. The capacitance increase is explained by a distorted ion shell model. The ion
salvation shell is perturbed such that it is capable of a closer approach of the ion and
the carbon surface. The discoveries at Drexel University that utilize the fine control to
create tailored porous carbon structures can maximize specific capacitance for a given
ultracapacitor system.
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