The production of 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% yield of hydrogen," Mallouk told attendees 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%," 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 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 expensive components—a titanium oxide electrode, a platinum
dark electrode, and iridium oxide catalyst. Substitutions for these are
necessary, and other researchers are working on solutions. A Massachusetts
Institute of Technology group is investigating cobalt and nickel catalysts, and
at Yale Univ.
and Princeton Univ. they are investigating manganese.
"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, 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.
"We could slow down major
recombination in the artificial system in the same way," said Mallouk.
"Electron transfer from the mediator to the dye would effectively outrun
the recombination reaction."
Currently the system uses only one photon
at a time, but a two-photon system, while more complicated, would be more
effective in using the full spectrum of sunlight.
Mallouk's main goal now is to track all
the energy pathways in his cell to understand the kinetics. Once he knows this,
he can model the cells and adjust portions to decrease energy loss and increase
efficiency.
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