Is future of fuel in photosynthesis?

PENN STATE (US) — Inexpensive hydrogen for automotive or jet fuel may be possible by mimicking photosynthesis, but the process needs to overcome several hurdles first.

“We are focused on the hardest way to make fuel,” says Thomas Mallouk, professor of materials chemistry and physics at Penn State.

“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.”

Splitting water into hydrogen and oxygen can be done in a variety of ways, but most are heavily energy intensive. The resultant hydrogen, which can be used to fuel vehicles or converted into a variety of hydrocarbons, inevitably costs more than existing fossil-based fuels.

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, but so far, is much less efficient than other solar energy conversion technologies.

The key to direct conversion is electrons. 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.

Mallouk presented his study at the annual meeting of the American Association for the Advancement of Science.

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

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,” says Mallouk. “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.”

Researchers don’t have a fix for the oxidation, so their catalysts and other molecules used in the cell structure eventually degrade, limiting the life of the solar fuel cell.

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

Substitutions for these are necessary, and other researchers are working on solutions, including cobalt and nickel catalysts.

“Cobalt and nickel don’t work as well as iridium, but they aren’t bad,” said Mallouk. “The cobalt work is spreading to other institutions as well.”

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, Mallouk says.

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.

“We could slow down major recombination in the artificial system in the same way,” says Mallouk. “Electron transfer from the mediator to the dye would effectively outrun the recombination reaction.”

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