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Working toward new energy with electrochemistry
Posted on Monday, August 20, 2007 @ 21:13:21 UTC by vlad
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In an effort to develop alternative energy sources such as fuel cells
and solar fuel from “artificial” photosynthesis, scientists at the U.S.
Department of Energy’s Brookhaven National Laboratory are taking a
detailed look at electrons – the particles that set almost all chemical
processes in motion.
Electron transfer plays a
vital role in numerous biological processes, including nerve cell
communication and converting energy from food into useful forms. It’s
the initial step in photosynthesis
as well, where charges are first separated and the energy is stored for
later use – one of the concepts behind energy production using solar
cells.
Understanding and controlling
the movement of electrons through individual molecules also could allow
for the development of new technologies such as extremely small
circuits, or help scientists find catalysts that give fuel cells a
much-needed boost in efficiency and affordability. Three Brookhaven
chemists will discuss how these applications are related to their most
recent findings at the 234th National Meeting of the American Chemical
Society. The details of their research are highlighted below.
A Different Way to Turn Water into Fuel
Brookhaven chemist
James Muckerman works with a team of researchers to design catalysts
inspired by photosynthesis, the natural process by which green plants
convert sunlight, water, and carbon dioxide into oxygen and
carbohydrates. The goal is to design a bio-inspired system that can
produce fuels like methanol or hydrogen directly from carbon dioxide or
water, respectively, using renewable solar energy. To replicate one of
the important steps in natural photosynthesis, Muckerman uses molecular
complexes containing the metal ruthenium as catalysts to drive the
conversion of water into oxygen, protons, and electrons. Specifically,
Muckerman’s group has set out to determine the electronic activity of a
catalyst recently developed in Japan. Unlike previous ruthenium
catalysts, which have a very short life, this catalyst has quinone
ligands attached to each of its ruthenium centers. These
electron-accepting molecules appear to make the catalyst very active
and stable. The challenge is to determine exactly how the catalyst
works.
“It was a controversial result,” said Muckerman, who compares the
lab results to calculations based on theory. “I believe that the
reaction occurs by ruthenium-mediated electron transfer from water
molecules bound to the metal centers to the quinone ligands. These
electron transfers are initiated by proton transfers from the bound
water moieties to the aqueous solution. The ruthenium atoms maintain
the same charge state during the entire catalytic cycle, indicating
that this catalyst works in a totally different way than the other
catalysts.”
This result could open up a new direction for designing future catalysts.
Revealing a Jumpstart in Organic Electron Transfer
Using organic molecules as electronic components in nanoscale
devices could lead to various technological advances including
small-scale circuits for improving solar cells. One of the most
important issues in this field is the role of molecule-metal contact
and the electron transfer that occurs between the two. With this idea
in mind, Brookhaven chemist Marshall Newton and former Brookhaven
research associate Vasili Perebeinos studied the electronic activity
involved in the self-assembly of sulfur-capped organic molecules
supported on a gold surface. Their results were surprising:
“The bottom line is that the electrical action in the formation of
this interface has already happened within the organic layer, without
direct involvement of the metal,” said Newton, who develops models to
understand these interactions in molecular systems. “That’s somewhat
unexpected because people typically say that the big electrical action
involves charge moving from or between the organic part and the metal
surface. But in this case, the electronic rearrangement occurs
internally during the process of bringing all of these organic chains
together before they are in contact with the metal.”
Newton believes this phenomenon is caused by the need to reduce electrical repulsions between the side-by-side organic chains.
An Affordable Alternative for Fuel Cells
Platinum is the most efficient metal electrocatalyst for
accelerating chemical reactions in fuel cells. However, the reactions
caused by the expensive metal are slow, and undesired side reactions
often degrade the electrode. In an effort to find an affordable
alternative with high activity and stability, Brookhaven chemist Ping
Liu and her research group are introducing ruthenium oxide to the
electronic system. By carefully forming just one thin layer of platinum
on a ruthenium-oxide surface, Ping has calculated that the
oxidation-reduction reaction (the driving force for fuel cells) happens
almost as quickly as with a pure platinum catalyst, while using much
less of the pricey metal and preventing its dissolution.
“Theoretically, when there’s one monolayer of platinum on
ruthenium-oxide, it has very close activity to pure platinum,” Liu
said. “It’s not quite as good, but it’s very close. This surface should
be one of the alternatives we consider for oxidation-reduction
catalysts.”
Future research plans include looking for ways to modify the
surface, adding other elements or metals, and further reducing the cost
by searching for a surface material less expensive than ruthenium
oxide.
Source: Brookhaven National Laboratory
Via: http://www.physorg.com/news106833052.html
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