Source: Proceedings of the National Academy of Sciences (vol 97, p 7687)
FOR years, polystyrene foam has been keeping our coffee hot and our beer
cold. Now chemists have taught single molecules of this plastic a more
impressive trick--how to snatch and store energy from light in the way that
plants do.
The researchers, based as the Los Alamos National Laboratory in New Mexico,
are aiming to build a new breed of cheap polystyrene-based batteries. They
say the batteries could be used to provide power for anything from cars to
cellphones.
The project is part of continuing efforts by Tom Meyer and his colleagues
at Los Alamos to find simple materials that can mimic the complex chemistry
of photosynthesis, which allows plants to store energy from sunlight in
chemical bonds.
The first step in photosynthesis is the absorption of a photon--a particle
of light--by an "antenna" composed of the green pigment chlorophyll. This
energy is then used to propel electrons around the chemical bonds of the
molecules involved in the reaction, ultimately stripping electrons from
water to make oxygen, and adding them to carbon dioxide to create
energy-rich sugars.
Meyer's team had already managed to recreate each of these steps
separately, using different synthetic molecules that work in conjunction
with an atom of the metal ruthenium. In his latest experiments, he decided
to link these molecules on a polymer backbone. "Because polystyrene
polymers are so widely used, their chemistry is very well understood," he
says. "So it was an obvious choice for the backbone."
To test out the new molecule, the researchers zapped a solution of it with
a laser. As they had hoped, some side chains containing the ruthenium
antennas were able to absorb the light energy and pass it along the polymer
chain. When the energy reached another type of ruthenium complex, it was
able to do some chemical work, pushing an electron to a different part of
the complex. Mobilising electrons in this way is what batteries
do--supplying electrons from an anode that can be drawn around a circuit
and back towards its cathode.
Michael Wasielewski, a photosynthetic chemist at Northwestern University in
Chicago, thinks Meyer's choice of polymer was the key. "All the right
properties are there," he says. "And we already have robust [manufacturing]
technology to work with polystyrene. It's a nice piece of work."
Because polystyrene derivatives are inexpensive to create, Meyer thinks his
polymer holds great promise for a wide array of devices--such as
solar-powered fuel cells that pull electrons from water to create hydrogen
fuel or to generate an electric current.
For now, his team's main aim is to increase the efficiency with which the
molecules convert energy, which is now hovering around 15 per cent.
"There's a lot of hard work ahead, but you have to expect that," he says.
"After all, it took natural photosynthesis more than a billion years to
evolve."
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