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This proof-of-concept version of the photoelectrochemical cell, which was used for laboratory tests, contains a photoactive solution made up of a mix of self-assembling molecules (in a glass cylinder held in place by metal clamp) with two electrodes protruding from the top, one made of platinum (the bare wire) and the other of silver (in a glass tube). Photo: Patrick Gillooly
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Plants are good at doing what scientists
and engineers have been struggling to do for decades: converting sunlight into
stored energy, and doing so reliably day after day, year after year. Now some
MIT scientists have succeeded in mimicking a key aspect of that process.
One of the problems with harvesting
sunlight is that the sun’s rays can be destructive to many materials. Sunlight
leads to a gradual degradation of many systems developed to harness it. But
plants have adopted an interesting strategy to address this issue: They
constantly break down their light-capturing molecules and reassemble them from
scratch, so the basic structures that capture the sun’s energy are, in effect,
always brand new.
That process has now been imitated by
Michael Strano, the Charles and Hilda Roddey Associate Professor of Chemical
Engineering, and his team of graduate students and researchers. They have
created a novel set of self-assembling molecules that can turn sunlight into
electricity; the molecules can be repeatedly broken down and then reassembled
quickly, just by adding or removing an additional solution. Their paper on the
work was published in Nature Chemistry.
Strano says the idea first occurred to
him when he was reading about plant biology. “I was really impressed by how
plant cells have this extremely efficient repair mechanism,” he says. In full
summer sunlight, “a leaf on a tree is recycling its proteins about every 45
minutes, even though you might think of it as a static photocell.”
One of Strano’s long-term research goals
has been to find ways to imitate principles found in nature using
nanocomponents. In the case of the molecules used for photosynthesis in plants,
the reactive form of oxygen produced by sunlight causes the proteins to fail in
a very precise way. As Strano describes it, the oxygen “unsnaps a tether that
keeps the protein together,” but the same proteins are quickly reassembled to
restart the process.
This action all takes place inside tiny
capsules called chloroplasts that reside inside every plant cell—and which is
where photosynthesis happens. The chloroplast is “an amazing machine,” Strano
says. “They are remarkable engines that consume carbon dioxide and use light to
produce glucose,” a chemical that provides energy for metabolism.
To imitate that process, Strano and his
team, supported by grants from the MIT Energy Initiative, the Eni Solar
Frontiers Center at MIT and the Department of Energy, produced synthetic
molecules called phospholipids that form disks; these disks provide structural
support for other molecules that actually respond to light, in structures
called reaction centers, which release electrons when struck by particles of
light. The disks, carrying the reaction centers, are in a solution where they attach
themselves spontaneously to carbon nanotubes. The nanotubes hold the
phospholipid disks in a uniform alignment so that the reaction centers can all
be exposed to sunlight at once, and they also act as wires to collect and
channel the flow of electrons knocked loose by the reactive molecules.
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From left to right, Associate Professor Michael Strano with graduate student Ardemis Boghossian and postdoctoral fellow Moon-Ho Ham, in one of the labs where they carried out their experiments.
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The system Strano’s team produced is
made up of seven different compounds, including the carbon nanotubes, the
phospholipids, and the proteins that make up the reaction centers, which under
the right conditions spontaneously assemble themselves into a light-harvesting
structure that produces an electric current. Strano says he believes this sets
a record for the complexity of a self-assembling system. When a surfactant is
added to the mix, the seven components all come apart and form a soupy
solution. Then, when the researchers removed the surfactant by pushing the
solution through a membrane, the compounds spontaneously assembled once again
into a perfectly formed, rejuvenated photocell.
“We’re basically imitating tricks that
nature has discovered over millions of years”—in particular, “reversibility,
the ability to break apart and reassemble,” Strano says. The team, which
included postdoctoral researcher Moon-Ho Ham and graduate student Ardemis
Boghossian, came up with the system based on a theoretical analysis, but then
decided to build a prototype cell to test it out. They ran the cell through
repeated cycles of assembly and disassembly over a 14-hour period, with no loss
of efficiency.
Strano says that in devising novel
systems for generating electricity from light, researchers don’t often study
how the systems change over time. For conventional silicon-based photovoltaic
cells, there is little degradation, but with many new systems being developed—either
for lower cost, higher efficiency, flexibility, or other improved
characteristics—the degradation can be very significant. “Often people see,
over 60 hours, the efficiency falling to 10% of what you initially saw,” he
says.
The individual reactions of these new
molecular structures in converting sunlight are about 40% efficient, or about
double the efficiency of today’s best solar cells. Theoretically, the
efficiency of the structures could be close to 100%, he says. But in the
initial work, the concentration of the structures in the solution was low, so
the overall efficiency of the device—the amount of electricity produced for a
given surface area—was very low. They are working now to find ways to greatly
increase the concentration.
Philip Collins ’90, associate professor
of experimental and condensed-matter physics at the University
of California, Irvine, who was not involved in this work,
says, “One of the remaining differences between man-made devices and biological
systems is the ability to regenerate and self-repair. Closing this gap is one
promise of nanotechnology, a promise that has been hyped for many years.
Strano's work is the first sign of progress in this area, and it suggests that
‘nanotechnology’ is finally preparing to advance beyond simple nanomaterials
and composites into this new realm.”
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