In capacitor research, much effort has been
focused on development of high energy density devices. It’s easy to see why.
The need for efficient, high-performance materials for electrical energy
storage has been growing in parallel with the ever-increasing demand for
electrical energy in mobile applications. Along the way dielectric capacitors,
which comprise an insulating material sandwiched between two conducting metal
plates, have become key components of portable electronics, computing systems
and electric vehicles. One hurdle preventing even faster growth is that in
contrast to batteries, which offer high storage capacity but slow delivery of
energy, capacitors provide fast delivery but poor storage capacity.
To close that gap the current strategy has
been to increase the specific surface area of the electrodes, employing
nanomaterials with large specific surface areas, which promise to deliver
electrical energy storage devices with a high energy density. Unfortunately,
these approach strategies to enhance the performance of dielectric capacitors
has not been able to simultaneously achieve large capacitance and high
breakdown voltage.
Recently a group of researchers at the
University of Delaware (UD) and the Chinese Academy of Sciences demonstrated
how such limitations can be overcome. In their paper, “Dielectric Capacitors
with Three-Dimensional Nanoscale Interdigital Electrodes for Energy Storage,”
(co-authored by Fangming Han, Guowen Meng, Fei Zhou, Li Song, Xinhua Li, Xiaoye
Hu, Xiaoguang Zhu, Bing Wu and Bingqing Wei) published in Science Advances, an
online-only journal of the American Association for the Advancement of Science
(AAAS), the researchers discuss how they used nanotechnology to design
electrodes that address the low ability of dielectric capacitors to store
energy.
To accomplish their goal the researchers
embedded carbon nanotubes in a uniquely designed and structured 3D
architecture. They fabricated a unique nanoporous—meaning the size of the pores
is generally 100 nanometers or smaller – anodic aluminum oxide (AAO) membrane
with two sets of interdigitated electrodes (here, to understand the concept you
should think of interwoven fingers between two hands with gloves, thus
increasing the ability of the capacitor to store an electrical charge). The
researchers deposited carbon nanotubes in both sets of pores inside the AAO
membrane, realizing a dielectric capacitor with 3D nanoscale interdigital
electrodes. The large specific surface area of the AAO membrane contributes to
the enhanced capacitance.
Bingqing Wei, professor of mechanical
engineering at UD noted that with this approach the researchers achieved an
energy density of about two watt-hours per kilogram, which is significantly
higher than that of other dielectric capacitor structures thus far reported and
is close to the value of a supercapacitor, which means it has promising
potential in high-density electrical energy storage for various applications.
The newly structured capacitor was
fabricated by Chemical Vapor Deposition (CVD) growth of carbon nanotubes (CNTs)
in the two sets of nanopores of the uniquely structured AAO membrane. The
nanoporous AAO is formed by electrochemical oxidation of aluminum in acidic
solutions (see Fig. 1, below)
Figure. 1 Schematic depiction of the
structure, fabrication process, and energy storage mechanism of the newly
developed dielectric capacitor. (A) Dielectric capacitor with 3D interdigital
electrode. (B) Breakdown structure of the dielectric capacitor. (C) Fabrication
process of the uniquely structured AAO membrane. (D) Schematic depiction of the
energy storage mechanism of a unit cell in the newly structured dielectric
capacitor from side view (top) and top view (bottom). (Source: University of
Delaware).
Another significant feature of the
capacitors is that the unique new three-dimensional nanoscale electrode also
offers high voltage breakdown, via the use of hemispheric barrier layers.
Simply put, this means that the integrated dielectric material (alumina, Al2O3)
does not easily fail in its intended function as an insulator.
In summary a capacitance density of about
47 μF/cm2 for 6-μm-thick HA-AAO was achieved and a breakdown voltage of about
15 V was observed, the researchers reported. Wei et al. anticipate that their
work will open up a new window to designing 3D nanoarchitectural electrodes for
various energy storage applications.
Also working to find a single dielectric
material able to maximize permittivity, breakdown strength, energy density and
energy extraction efficiency Joseph Perry, professor in the School of Chemistry
and Biochemistry at Georgia Tech’s Center for Organic Photonics and Electronics
(COPE)and his colleagues reported in July in the journal Advanced Energy
Materials that they have developed a new capacitor dielectric material said to
provide an electrical energy storage capacity rivaling certain batteries, with
both a high energy density and high power density.
Using a hybrid silica sol-gel material and
self-assembled monolayers of a common fatty acid (octylphosphonic acid, which
provides insulating properties), the researchers demonstrated maximum
extractable energy densities up to 40 joules per cubic centimeter, an energy
extraction efficiency of 72 percent at a field strength of 830 volts per
micron, and a power density of 520 watts per cubic centimeter. This performance
exceeds that of conventional electrolytic capacitors and thin-film lithium ion
batteries, though it doesn’t match the lithium ion battery formats commonly
used in electronic devices and vehicles.
By depositing a nanoscale self-assembled
monolayer of n-octylphosphonic acid less than a nanometer thick on top of the
hybrid sol-gel, the monolayer serves as an insulating layer. Consequently the
bilayer structure is said to block the injection of electrons into the sol-gel
material, providing low leakage current and high breakdown strength.
If the material can be scaled up from
laboratory samples, devices made from it could surpass traditional electrolytic
capacitors for applications in electromagnetic propulsion, electric vehicles
and defibrillators, for example. Capacitors often complement batteries in these
applications because they can provide large amounts of current quickly. If the
scale-up is successful, Perry expects to commercialize the material through a
startup company or SBIR (Small Business Innovations Research) project.