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 (Feb. 17) 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.