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Solar cell directly splits water for hydrogen
Posted on Monday, February 18, 2008 @ 22:12:35 UTC by vlad
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Plants trees and algae do it. Even some bacteria and moss do it, but
scientists have had a difficult time developing methods to turn
sunlight into useful fuel. Now, Penn State researchers have a
proof-of-concept device that can split water and produce recoverable
hydrogen.
"This is a proof-of-concept
system that is very inefficient. But ultimately, catalytic systems with
10 to 15 percent solar conversion efficiency might be achievable," says
Thomas E. Mallouk, the DuPont Professor of Materials Chemistry and
Physics. "If this could be realized, water photolysis would provide a
clean source of hydrogen fuel from water and sunlight."
Although solar cells can now
produce electricity from visible light at efficiencies of greater than
10 percent, solar hydrogen cells – like those developed by Craig
Grimes, professor of electrical engineering at Penn State – have been
limited by the poor spectral response of the semiconductors used. In
principle, molecular light absorbers can use more of the visible
spectrum in a process that is mimetic of natural photosynthesis.
Photosynthesis uses chlorophyll and other dye molecules to absorb
visible light.
So far, experiments with natural and synthetic dye molecules have
produced either hydrogen or oxygen-using chemicals consumed in the
process, but have not yet created an ongoing, continuous process. Those
processes also generally would cost more than splitting water with
electricity. One reason for the difficulty is that once produced,
hydrogen and oxygen easily recombine. The catalysts that have been used
to study the oxygen and hydrogen half-reactions are also good catalysts
for the recombination reaction.
Mallouk and W. Justin Youngblood, postdoctoral fellow in chemistry,
together with collaborators at Arizona State University, developed a
catalyst system that, combined with a dye, can mimic the electron
transfer and water oxidation processes that occur in plants during
photosynthesis. They reported the results of their experiments at the
annual meeting of the American Association for the Advancement of
Science today in Boston.
The key to their process is a tiny complex of molecules with a
center catalyst of iridium oxide molecules surrounded by orange-red dye
molecules. These clusters are about 2 nanometers in diameter with the
catalyst and dye components approximately the same size. The
researchers chose orange-red dye because it absorbs sunlight in the
blue range, which has the most energy. The dye used has also been
thoroughly studied in previous artificial photosynthesis experiments.
They space the dye molecules
around the center core leaving surface area on the catalyst for the
reaction. When visible light strikes the dye, the energy excites
electrons in the dye, which, with the help of the catalyst, can split
the water molecule, creating free oxygen.
"Each surface iridium atom can cycle through the water oxidation
reaction about 50 times per second," says Mallouk. "That is about three
orders of magnitude faster than the next best synthetic catalysts, and
comparable to the turnover rate of Photosystem II in green plant
photosynthesis." Photosystem II is the protein complex in plants that
oxidizes water and starts the photosynthetic process.
The researchers impregnated a titanium dioxide electrode with the
catalyst complex for the anode and used a platinum cathode. They
immersed the electrodes in a salt solution, but separated them from
each other to avoid the problem of the hydrogen and oxygen recombining.
Light need only shine on the dye-sensitized titanium dioxide anode for
the system to work. This type of cell is similar to those that produce
electricity, but the addition of the catalyst allows the reaction to
split the water into its component gases.
The water splitting requires 1.23 volts, and the current
experimental configuration cannot quite achieve that level so the
researchers add about 0.3 volts from an outside source. Their current
system achieves an efficiency of about 0.3 percent.
"Nature is only 1 to 3 percent efficient with photosynthesis," says
Mallouk. "Which is why you can not expect the clippings from your lawn
to power your house and your car. We would like not to have to use all
the land area that is used for agriculture to get the energy we need
from solar cells."
The researchers have a variety of approaches to improve the
process. They plan to investigate improving the efficiency of the dye,
improving the catalyst and adjusting the general geometry of the
system. Rather than spherical dye catalyst complexes, a different
geometry that keeps more of the reacting area available to the sun and
the reactants might be better. Improvements to the overall geometry may
also help.
"At every branch in the process, there is a choice," says Mallouk.
"The question is how to get the electrons to stay in the proper path
and not, for example, release their energy and go down to ground state
without doing any work."
The distance between molecules is important in controlling the rate
of electron transfer and getting the electrons where they need to go.
By shortening some of the distances and making others longer, more of
the electrons would take the proper path and put their energy to work
splitting water and producing hydrogen.
Source: Penn State
Via: http://www.physorg.com/news122534699.html
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Re: Solar cell directly splits water for hydrogen (Score: 1)
by malc on Tuesday, February 19, 2008 @ 00:49:52 UTC
(User Info | Send a Message) http://web.ukonline.co.uk/mripley | Interesting. There is a definite need for a "mobile" fuel. The static domestic fuel is electricity in the absence of anything better, and we can generate our own electricity from solar and wind. However, "mobile" fuel for vehicles is a problem. It is clear that biofuels are a non starter to anyone with more brain cells than an amoeba so hydrogen would seem a good alternate fuel....or is it?
There is something else: compressed air. With a compressed air car the fuel is anything which can compress air so we are back being able to use electricity from solar and wind again. In fact there is the distinct possibility of using compressed air as the "battery" to store energy for the home as well.
So compressed air is potentially a universal domestic energy storage medium. Given that there is now a compressed air car it seems to me that some of the technical difficulties of air storage and pressure release have been solved. If you can make a car then running a generator for the home is EASY. |
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Costs of Solar Photovoltaic Panels Substantially Eclipse Benefits (Score: 1)
by vlad on Friday, February 22, 2008 @ 21:04:34 UTC
(User Info | Send a Message) http://www.zpenergy.com | Despite increasing popular support for solar photovoltaic panels in the
United States, their costs far outweigh the benefits, according to a
new analysis by Severin Borenstein, a professor at the University of
California, Berkeley's Haas School of Business and director of the UC
Energy Institute. "Solar photovoltaic (PV) is a very exciting
technology, but the current technology is not economic," said
Borenstein. "We are throwing money away by installing the current solar
PV technology, which is a loser." In his January working paper, "The
Market Value and Cost of Solar Photovoltaic Electricity Product,"
Borenstein also found that, even after considering that the panels
reduce greenhouse gases, their costs still far outweigh their social
benefits. The bottom line, Borenstein argues in his paper, is that
solar PV panels are not ready for widespread installation. Rather than
subsidizing residential solar PV installations, as many states do, he
favors more state and federal funding for research and development. "We
need a major scientific breakthrough, and we won't get it by putting
panels up on houses," he said in a recent interview on campus. "It is
going to come in the labs." Solar photovoltaic panels generate more
power on summer afternoons when the sun is shining most intensely,
which is also when the value of electricity is higher for most U.S.
electricity systems, Borenstein noted. Proponents of the devices have
pointed out that most previous analyses fail to address that fact.
Borenstein uses actual wholesale electricity prices and simulated data
to calculate how much that timing enhances the value of solar
photovoltaic panels. He found that the favorable timing of solar PV
production increases its value by up to 20 percent. However, the
premium value of solar PV could be from 30 percent to 50 percent higher
if U.S. systems were run with less capacity and prices were allowed to
rise as demand increases at different times of the day, said
Borenstein, who has long advocated for such variable time pricing. He
noted that U.S. systems typically operate with excess capacity and that
consumers pay the same price for electricity at all times of the day. -
Source [www.newswise.com]
Via: http://www.keelynet.com/#whatsnew [www.keelynet.com]
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