between the two electrodes is a polymer film that acts as a reservoir of charged ions,
similar to the role of the electrolyte paste in a battery. When the electrodes are pressed
together, the polymer oozes into the tiny pores in much the same way that melted
cheese soaks into the nooks and crannies of artisan bread in a panini. When the poly-
mer cools and solidifies, it forms an extremely strong mechanical bond.
¡°The biggest problem with designing load-bearing supercaps is preventing them from
delaminating,¡± said Westover. ¡°Combining nanoporous material with the polymer
electrolyte bonds the layers together tighter than superglue.¡±
The use of silicon in structural supercapacitors is best suited for consumer electron-
ics and solar cells, but Pint and Westover are confident that the rules that govern the
load-bearing character of their design will carry over to other materials, such as carbon
nanotubes and lightweight porous metals like aluminum.
The intensity of interest in ¡®multifunctional¡¯ devices of this sort is reflected by the fact
that the U.S. Department of Energy¡¯s Advanced Research Project Agency for Energy
is investing $8.7 million in research projects that focus specifically on incorporating
energy storage into structural materials. There have also been recent press reports of
several major efforts to develop multifunctional materials or structural batteries for use
in electric vehicles and for military applications. However, Pint pointed out that there
have not been any reports, in the technical literature, of tests performed on structural
energy storage materials that show how they function under realistic mechanical loads.
Amrutur Anilkumar, professor of the practice in mechanical engineering, postdoctoral
associate Shahana Chatterjee, graduate student Landon Oakes, undergraduate mechan-
ical engineering majors John Tian, Shivaprem Bernath and Farhan Nur Shabab and
high school student Rob Edwards collaborated in the project.
The research was supported by National Science Foundation grants CMMI 1334269 and
EPS 104083. Materials fabrication was conducted in part at the Center for Nanophase
Materials Sciences at Oak Ridge National Laboratory that is supported by the Office of
Basic Energy Sciences of the U.S. Department of Energy.
(Courtesy National Science Foundation, www.nsf.gov & news.vanderbilt.edu)
Structural Energy Storage Dream Becomes Reality
Robust Supercapacitor Opens Door To ¡°World Of Possibilities¡±
Steve Winwood at
Bluegrass... Pg 12
The Sacred Herb
Hyssop... Pg 8
& more... Pg 14
A weekend of free
fishing... Pg 10
Liberating devices from their power cords
by David Salisbury, May 19, 2014: Imagine a future in which
our electrical gadgets are no longer limited by plugs and
external power sources.
This intriguing prospect is one of the reasons for the current
interest in building the capacity to store electrical energy di-
rectly into a wide range of products, such as a laptop whose
casing serves as its battery, or an electric car powered by en-
ergy stored in its chassis, or a home where the dry wall and
siding store the electricity that runs the lights and appliances.
It also makes the small, dull grey wafers, that graduate stu-
dent Andrew Westover and Assistant Professor of Mechani-
cal Engineering Cary Pint have made in Vanderbilt¡¯s Nano-
materials and Energy Devices Laboratory, far more
2014 JUNE #5-5
important than their nondescript appearance suggests.
¡°These devices demonstrate, for the first time as far as we can tell, that it is possible to
create materials that can store and discharge significant amounts of electricity while
they are subject to realistic static loads and dynamic forces, such as vibrations or
impacts,¡± said Pint. ¡°Andrew has managed to make our dream of structural energy
storage materials into a reality.¡±
That is important because structural energy storage will change the way in which a
wide variety of technologies are developed in the future. ¡°When you can integrate
energy into the components used to build systems, it opens the door to a whole new
world of technological possibilities. All of a sudden, the ability to design technologies
at the basis of health, entertainment, travel and social communication will not be lim-
ited by plugs and external power sources,¡± Pint said.
The new device that Pint and Westover has developed is a supercapacitor that stores
electricity by assembling electrically charged ions on the surface of a porous material,
instead of storing it in chemical reactions the way batteries do. As a result, supercaps
can charge and discharge in minutes, instead of hours, and operate for millions of
cycles, instead of thousands of cycles like batteries.
In a paper appearing online May 19 in the journal Nano Letters, Pint and Westover
reported that their new structural supercapacitor operates flawlessly in storing and
releasing electrical charge while subject to stresses or pressures up to 44 psi and vibra-
tional accelerations over 80 g (significantly greater than those acting on turbine blades
in a jet engine).
Furthermore, the mechanical robustness of the device doesn¡¯t compromise its energy
storage capability. ¡°In an unpackaged, structurally integrated state our supercapacitor
can store more energy and operate at higher voltages than a packaged, off-the-shelf
commercial supercapacitor, even under intense dynamic and static forces,¡± Pint said.
One area where supercapacitors lag behind batteries is in electrical energy storage
capability: Supercaps must be larger and heavier to store the same amount of energy as
lithium-ion batteries. However, the difference is not as important when considering
multifunctional energy storage systems.
¡°Battery performance metrics change when you¡¯re putting
energy storage into heavy materials that are already needed
for structural integrity,¡± said Pint. ¡°Supercapacitors store ten
times less energy than current lithium-ion batteries, but they
can last a thousand times longer. That means they are better
suited for structural applications. It doesn¡¯t make sense to
develop materials to build a home, car chassis, or aerospace
vehicle if you have to replace them every few years because
they go dead.¡±
Westover¡¯s wafers consist of electrodes made from silicon
that have been chemically treated so they have nanoscale
pores on their inner surfaces and then coated with a protec-
tive ultrathin graphene-like layer of carbon. Sandwiched
Close-up of structural supercapacitor.
(Joe Howell / Vanderbilt)