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 light.

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 photosynthesis."

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 sunlight."

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. 

With hydrogen fuel looking more and more promising, Mallouk and Youngblood's research is certainly a significant breakthrough.