"Just think how often your fancy new mobile phone or computer has become little
more than a paperweight because the battery lost its zeal for doing its job,"
says John Chmiola, a chemist with the Lawrence
Berkeley National Laboratory (Berkeley Lab). "At a time when cellphones
can do more than computers could do at the beginning of the Clinton presidency,
it would be an understatement to say that batteries have not been holding up
their end of the mobile device bargain."
Chmiola is a staff scientist in the Advanced Energy Technologies Department
of Berkeley Lab's Environmental Energy Technologies Division. His research
is aimed at addressing this problem of relatively short-lived portable energy
storage devices. Chmiola believes he has found a solution in electrochemical
capacitors, which are commonly referred to as "supercapacitors"
because of their higher energy storage densities than conventional dielectric
capacitors and higher abuse tolerance than batteries.
Berkeley Lab chemist John Chmiola is developing a new breed of micro-supercapacitors that could substantially boost the performance and longevity of portable electric energy storage devices. (Photo by Roy Kaltschmidt, Berkeley Lab Public Affairs)
In a paper published in the April 23, 2010 issue of the journal Science, titled
"Monolithic Carbide-Derived Carbon Films for Micro-Supercapacitors,"
Chmiola and Yury Gogotsi of Drexel University, along with other co-authors,
describe a unique new technique for integrating high performance micro-sized
supercapacitors into a variety of portable electronic devices through common
microfabrication techniques.
By etching electrodes made of monolithic carbon film into a conducting substrate
of titanium carbide, Chmiola and Gogotsi were able to create micro-supercapacitors
featuring an energy storage density that was at least double that of the best
supercapacitors now available. When used in combination with microbatteries,
the power densities and rapid-fire cycle times of these micro-supercapacitors
should substantially boost the performance and longevity of portable electric
energy storage devices.
"The prospect of integrating batteries and supercapacitors with the micro-electromechanical
systems (MEMS) they power represents a conceptual leap forward over existing
methods for powering such devices," Chmiola says. "Furthermore,
since the same fabrication processes that produced the devices needing the electrical
energy also produced the devices storing that energy, we provide a framework
for potentially increasing the density of microelectronic devices and allowing
improved functionality, reduced complexity, and enhanced redundancy."
The two principal systems today for storing electrical energy are batteries
and supercapacitors. Batteries store electrical energy in the form of chemical
reactants and generally display even higher energy storage densities than supercapacitors.
However, the charging and discharging of a battery exact a physical toll on
electrodes that eventually ends the battery's life after several thousand
charge-discharge cycles. In supercapacitors, energy is stored as electrical
charge, which does not impact electrodes during operation. This allows supercapacitors
to be charged and discharged millions of times.
"We have known for some time that supercapacitors are faster and longer-lasting
alternatives to conventional batteries," Gogotsi says, "so we decided
to see if it would be possible to incorporate them into microelectronic devices
and if there would be any advantage to doing so."
Chmiola and Gogotsi chose titanium carbide as the substrate in this study because
while all metal carbides can be selectively etched with halogens so that a monolithic
carbon film is left behind, titanium carbide is readily available, relatively
inexpensive and can be used at the same temperatures as other microfabrication
processes.
"Plus, we have a body of work on titanium carbide precursor carbons that
provided us with a lot of data to draw from for understanding the underlying
science," Chmiola says.
The process started with titanium carbide ceramic plates being cut to size
and polished to a thinness of approximately 300 micrometers. The titanium was
then selectively etched from one face of the plate using chlorine at elevated
temperatures, a process that is similar to current dry-etching techniques for
MEMS and microchip fabrications.
Chlorinating the titanium removed the metal atoms and left in place a monolithic
carbon film, a material with a proven track record in supercapacitors produced
via the traditional "sandwich construction" technique.
"By using microfabrication techniques to produce our supercapacitors
we avoided many of the pitfalls of the traditional method," says Chmiola,
"namely poor contact between electro-active particles in the electrode,
large void spaces between particles that don't store charge, and poor
contact between the electro-active materials and the external circuitry."
The electrical charge storage densities of the micro-supercapacitors were measured
in two common electrolytes. As promising as the results were, Chmiola notes
the impressive figures were achieved without the "decades of optimization"
that other electronic devices have undergone. This, he says, "hints at
the possibility that the energy density ceiling for microfabricated supercapacitors
is, indeed, quite high."
Adds Gogotsi, "Given their practically infinite cycle life, micro-supercapacitors
seem ideal for capturing and storing energy from renewable resources and for
on-chip operations."
The next step of the work is to scale down the size of the electrodes and improve
the dry etching procedure for removing metal atoms from metal carbides to make
the process even more compatible with commercial microfabrication technology.
At Berkeley Lab, Chmiola is working on the development of new electrolytes that
can help increase the energy storage densities of his micro-supercapacitors.
He is also investigating the factors that control the usable voltage window
of different electrolytes at a carbon electrode.
"My ultimate goals are to increase energy stored to levels closer to
batteries, and preserve both the million-plus charge-discharge cycles and recharge
times of less than five minutes of these devices," says Chmiola. "I
think this is what the end users of portable energy storage devices really desire."
Co-authoring the Science paper with Chmiola and Gogotsi were Celine Largeot,
Pierre-Louis Taberna and Patrice Simon of Toulouse University in France.
Berkeley Lab is a U.S. Department of Energy national laboratory located in
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by the University of California. Visit our website at http://www.lbl.gov.