Obtaining fuel by mimicking photosynthesis
Inexpensive hydrogen for automotive or jet fuel may be possible by mimicking photosynthesis, according to a Penn State materials chemist, but a number of problems need to be solved first.
“We are focused on the hardest way to make fuel,” said Thomas Mallouk, Evan Pugh Professor of Materials Chemistry and Physics. “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. So far, it 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.
“Currently, we are getting only 2 to 3 percent yield of hydrogen,” Mallouk told attendees on Feb. 19 at the annual meeting of the American Association for the Advancement of Science. “For systems like this to be useful, we will need to get closer to 100 percent,” he added.
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,” said 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.”
So far, the researchers do not have a fix for the oxidation, so their catalysts and other molecules used in the cell structure eventually degrade, limiting the life of the .
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