Berkeley Lab scientists
delivered nearly 100 presentations at the American Chemical Society’s
Fall 2010 national meeting in Boston, August 22-26, 2010.
In the opening scene of the famous 1967 movie The Graduate, Benjamin Braddock,
at a party to celebrate his new degree, is given one word of advice for his
future: “Plastics.” Were young Benjamin to be receiving that advice
today the word might well have been: “Batteries.”
Economic forecasters say that the market for advanced batteries that can power
electric, hybrid-electric and the emerging plug-in hybrid electric vehicles
is going to be worth billions of dollars. Based on their performances in electronics
and power tools, lithium-ion batteries have the potential to be far superior
to nickel-metal hydride batteries but several technological issues must be addressed
before they’re applied to vehicles.
Marca Doeff, a chemist with Berkeley Lab’s Materials Sciences Division,
presented a talk titled “Advanced Li-ion battery cathode materials for
vehicle technologies.” She focused on the cathode as one of the most expensive
components in lithium-ion batteries.
“Also,” Doeff said, “the cathode is the determinant of energy
density in the cell because the capacity is typically much lower than that of
the graphite anode, with which it must be matched.”
Doeff and her colleagues are experimenting with various approaches for lowering
the cost and improving the performance of lithium-ion cathodes. Their studies
include partial substitution of the expensive cobalt constituent with aluminum,
titanium or iron in layered mixed transition metal oxides now used in batteries.
So far they have found that a five-percent substitution of cobalt with aluminum
increases cathode performance and cycle stability. Substitution with small amounts
of titanium also led to the formation of a high-capacity and high-rate positive
electrode material, whereas substitution with iron led to lower cathode capacities
and poor rate capabilities.
“Our work shows that changes in electrochemical performance of the cathode
depend highly on the nature of the substituting atom and its effect on the crystal
structure,” Doeff said.
High Efficiency Solar Cells and other Nano Delights
At that same graduation party today, young Ben Braddock might also have been
told to think “Nano.” Economic forecasters foresee an even more
bountiful future for nanoscale materials, particularly in solar energy and the
electronics fields. “Nanoscale electronic materials: Challenges and opportunities,”
was the title of a talk by Ali Javey, a faculty scientist in Berkeley Lab’s
Materials Sciences Division. In his talk Javey described a technique for engineering
arrays of nanoscale pillars for a broad range of applications, including low
cost, highly efficient solar cells, and artificial skin that provides prosthetic
limbs with the sense of touch.
“Our technique provides large-scale assembly of highly ordered and regular
arrays of nanowire components on flexible substrates through a simple contact
printing process,” Javey said. “The ability to interface nanowire
sensors with integrated electronics on large scales and with high uniformity
presents an important advance toward the integration of nanomaterials for sensor
applications.”
This technology is being applied to portable electronic and wearable human
interface applications, including artificial skin. The idea is that with the
integration of advanced prosthetics into the brain for better control of joints,
the addition of electronic skin with nanowire sensors could enable patients
to regain their sense of touch. The skin might also be used in robotics, governing
how much pressure a robot applies to an object.
“Our mechanically flexible, artificial skin sensor provides impressive
mechanical robustness and electrical properties,” Javey said.
Additionally, by utilizing optically active nanowire sensors, Javey and his
colleagues have been able to produce highly regular, single-crystalline nanopillar
arrays of semiconductors on aluminum substrates that were then configured as
solar cell modules.
“Through experiments and modeling, we’ve demonstrated the that
we can configure these solar modules on both rigid and flexible substrates with
enhanced carrier collection efficiency and broadband photo absorption arising
from the geometric configuration of the nanopillars,” Javey said. “This
is a hugely promising to lower the cost of efficient solar cells.”
Hydrogen from Sunlight Via Qdot-Seeded Nanorods
One word of advice that the graduate might not have been given at the party
is “hydrogen.” While experts agree that hydrogen could command a
key role in future renewable energy technologies, a relatively cheap, efficient
and carbon-neutral means of producing it must first be developed. The photocatalytic
production of hydrogen from water using solar energy meets all the necessary
criteria but there remain many materials-related obstacles to the widespread
use of this approach. Berkeley Lab Director Paul Alivisatos, a chemist and leading
authority on nanotechnology for energy discussed one idea for overcoming some
of these obstacles in his talk titled “Photocatalytic hydrogen production
with tunable nanorod heterostructures.”
In this talk, Alivisatos described a model nanosystem in which a quantum dot
seed of cadmium selenide, a hydrogen-generating catalyst, is embedded inside
a platinum-tipped cadmium sulfide nanorod.
“In such structures, holes are three-dimensionally confined to the cadmium
selenide, whereas the delocalized electrons are transferred to the metal tip,”
Alivisatos said. “Consequently, the electrons are separated from the holes
over three different components and by a tunable physical length.”
The seeded nanorod metal tip facilitates efficient long-lasting charge carrier
separation and minimizes back reaction of intermediates. By tuning the nanorod
heterostructure length and the seed size, Alivisatos and his group are able
to significantly increase hydrogen production compared to that of unseeded rods.
“We found our a multi-component nanoheterostructures to be highly active
for hydrogen production, with an apparent quantum yield of 20-percemt at 450
nanometers,” Alivisatos said. “Our systems were active under orange
light illumination and demonstrated improved stability compared to seedless
cadmium sulfide nanorods.”