Mimicking photosynthesis key to inexpensive solar-powered jet fuel

Monday, February 21, 2011

WASHINGTON - Scientists at the Penn State University have said that making cheap hydrogen for automotive or jet fuel is possible by mimicking photosynthesis.

“We are focused on the hardest way to make fuel. We are creating an artificial system that mimics photosynthesis, but it will be practical only when it is as cheap as gasoline or jet fuel,” said Thomas Mallouk, Evan Pugh Professor of Materials Chemistry and Physics.

While some researchers have used solar cells to make electricity or use concentrated solar heat to split water, Mallouk’s process uses the energy in blue light directly. So far, it is much less efficient than other solar energy conversion technologies.

Like the dyes that naturally occur in plants, inorganic dyes absorb sunlight and the energy kicks out an electron. Left on its own, the electron would recombine creating heat, but if the electrons can be channeled-molecule-to-molecule-far enough away from where they originate, the electrons can reach the catalyst and split the hydrogen from the oxygen in water.

“Currently, we are getting only 2 to 3 percent yield of hydrogen. For systems like this to be useful, we will need to get closer to 100 percent,” he said.

But recombination of electrons is not the only problem with the process. The oxygen-evolving end of the system is a chemical wrecking ball and this means the lifetime of the system is currently limited to a few hours.

“The oxygen side of the cell is making a strong oxidizing agent and the molecules near can be oxidized. Natural photosynthesis has the same problem, but it has a self-repair mechanism that periodically replaces the oxygen-evolving complex and the protein molecules around it,” said Mallouk.

Currently, the researchers are using only blue light, but would like to use the entire visible spectrum from the sun. They are also using expensive components - a titanium oxide electrode, a platinum dark electrode and iridium oxide catalyst.

While the designed structure of the fuel cell directs many of the electrons to the catalyst, most of them still recombine, giving over their energy to heat rather than chemical bond breaking.

The manganese catalysts in photosystem II-the photosynthesis system by which plants, algae and photosynthetic bacteria evolve oxygen-are just as slow as ours, said Mallouk.

Photosystem II works efficiently by using an electron mediator molecule to make sure there is always an electron available for the dye molecule once it passes its current electron to the next molecule in the chain.

The process was described at the annual meeting of the American Association for the Advancement of Science. (ANI)

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