Photosynthesis is the fundamental energy capture process
which forms the foundation of all life on Earth. On a most basic level,
it involves using sunlight to split water molecules into hydrogen and oxygen
and then using the hydrogen captured to fuel sugar production. With
hydrogen becoming more popular as a possible alternative fuel source, many
researchers have yearned to duplicate this most basic of natural processes to
allow for cheap,
efficient hydrogen production. They had little success -- until now.
In the past, natural and synthetic dye molecules which tried to split hydrogen
and water were consumed during the reactions and did not provide a
sustained reaction. Worse yet, the chemical reactions were often from a
net perspective endothermic; in other words they required energy instead of
producing it. Part of this is because of the ease with which oxygen and
hydrogen recombine, and the fact that most of these investigated catalysts also
catalyze the recombination, destroying your products.
Thomas Mallouk, a DuPont Professor of Materials Chemistry and Physics, and
W. Justin Youngblood, postdoctoral fellow in chemistry, together with
collaborators at Arizona State University succeeded where others
have failed. The researchers developed a dye/catalyst system that mimics
the oxidative and electron transfer processes of photosynthesis, ultimately
producing hydrogen gas. Their findings were presented at the meeting
of the American Association for the Advancement of Science today in Boston.
Clusters of molecules using iridium oxide molecules as a center catalyst,
surrounded by light absorbing orange-red dye molecules comprise the finished
product. The 2 nm complexes are roughly half dye and half catalyst in
terms of diameter. Orange-red dye was selected due to its extensive
experimental record and its ability to absorb high energy blue wavelength
Water molecules bond to the complex, and when the complex absorbs sunlight, it splits
them into hydrogen and oxygen. Mallouk enthuses upon its near
biological efficiency, stating, "Each surface iridium atom can cycle
through the water oxidation reaction about 50 times per second. 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
The process needs a tiny bit of juice to get started. The voltage
required to split water is 1.23 V, and the system is almost at this power level.
By adding 0.3 V from titanium dioxide anode and platinum cathode
electrodes, the water begins to split. Separating the electrodes
effectively reduces hydrogen/oxygen recombination.
The current process has a positive efficiency of about 0.3 percent. This
sounds pretty measly, but as Mallouk puts it, "Nature is only 1 to 3
percent efficient with photosynthesis. Which is why you cannot 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."
Mallouk hopes to eventually achieve efficiencies better
than that of natural processes. By changing the molecular geometry,
he plans on upping the efficiency by better allowing light to be absorbed or by
improving the bonding of water molecules to the surface of the complex.
Mallouk states optimistically, "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. If this could be realized,
water photolysis would provide a clean source of hydrogen fuel from water and
The fact that the efficiency is anywhere near that of the Photosystem II
protein complex, a marvel of biological design, is impressive in itself.
The fact that this system could be competitive one day with modern solar
technology (currently around 10 percent efficient) and help
to replace fossil fuels is even more impressive.
fuel looking more and more promising, Mallouk and Youngblood's research is
certainly a significant breakthrough.