“This is a proof-of-concept system that is very inefficient. But ultimately, catalytic systems with 10 to 15 per cent solar conversion efficiency might be achievable. If this could be realised, water photolysis would provide a clean source of hydrogen fuel from water and sunlight,” says Thomas E Mallouk, the DuPont Professor of Materials Chemistry and Physics.

Working in collaboration with collaborators at Arizona State University, Mallouk and W Justin Youngblood, postdoctoral fellow in chemistry, developed a catalyst system that, combined with a dye, could mimic the electron transfer and water oxidation processes that occur in plants during photosynthesis.

Presenting their work at the annual meeting of the American Association for the Advancement of Science in Boston, the researchers said that the key to their process was a tiny complex of molecules with a centre catalyst of iridium oxide molecules surrounded by orange-red dye molecules.

The researchers said that such clusters were about two nanometres in diameter with the catalyst and dye components approximately the same size.

As regards the selection of orange-red dye, the researchers said that it was chosen because it could absorb sunlight in the blue range, which has the most energy. The dye used had also been thoroughly studied in previous artificial photosynthesis experiments, they said.

The researchers say that when visible light strikes the dye, the energy excites electrons in the dye, which can split the water molecule with the help of the catalyst, and create free oxygen.

“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,” says Mallouk.

Photosystem II is the protein complex in plants that oxidises water, and starts the photosynthetic process.

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 per cent.

“Nature is only one to three per cent 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,” says Mallouk.

The researchers are employing a variety of strategies to improve the process.