Announcement

Collapse
No announcement yet.

Solar Cell, Heal Thyself

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • Solar Cell, Heal Thyself

    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: http://web.mit.edu/newsoffice/2010/self-healing-solar.html
    Every minute of your life is important.




    From: A. Papazian
Working...
X