Massachusetts Institute of Technology
Solar cell, heal thyself
New self-assembling photovoltaic technology can keep repairing itself
to avoid any loss in performance. David L. Chandler, MIT News Office
August 2, 2010 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 September 7, 2010
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 highly 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=3B 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 on Sept. 5 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=3B 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 – wire-like hollow tubes of carbon atoms that are
a few billionths of a meter thick yet stronger than steel and capable
of conducting electricity a thousand times better than copper. 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.
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 – similar in principle to the chemicals that
BP has sprayed into the Gulf of Mexico to break apart oil – 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 percent of
what you initially saw,’ he says.
The individual reactions of these new molecular structures in
converting sunlight are about 40 percent efficient, or about double
the efficiency of today’s best solar cells. Theoretically, the
efficiency of the structures could be close to 100 percent, 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.’
>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. Photo: Patrick
Gillooly
Source:
Every minute of your life is important.
From: A. Papazian
http://web.mit.edu/newsoffice/2010/self-healing-solar.html