A scanning electron microscope image of tiny silicon pillars, used to absorb light. When dotted with the new catalyst and exposed to sunlight, these pillars efficiently generate hydrogen gas from the hydrogen ions liberated by splitting water. Each pillar is approximately two micrometers in diameter. (Image courtesy of Christian D. Damsgaard, Thomas Pedersen and Ole Hansen, Technical University of Denmark.) |
These
studies led them to related compounds, which eventually took them to
molybdenum sulfide. “Molybdenum is an inexpensive solution” for
catalyzing hydrogen production, Chorkendorff said.
Scientists
have engineered a cheap, abundant alternative to the expensive platinum
catalyst and coupled it with a light-absorbing electrode to make
hydrogen fuel from sunlight and water.
The
discovery is an important development in the worldwide effort to mimic
the way plants make fuel from sunlight, a key step in creating a green
energy economy. It was reported last week in Nature Materials
by theorist Jens Nørskov of the Department of Energy’s SLAC National
Accelerator Laboratory and Stanford University and a team of colleagues
led by Ib Chorkendorff and Søren Dahl at the Technical University of
Denmark (DTU).
Hydrogen
is an energy dense and clean fuel, which upon combustion releases only
water. Today, most hydrogen is produced from natural gas which results
in large CO2-emissions. An alternative, clean method is to make hydrogen
fuel from sunlight and water. The process is called
photo-electrochemical, or PEC, water splitting. When sun hits the PEC
cell, the solar energy is absorbed and used for splitting water
molecules into its components, hydrogen and oxygen.
Progress
has so far been limited in part by a lack of cheap catalysts that can
speed up the generation of hydrogen and oxygen. A vital part of the
American-Danish effort was combining theory and advanced computation
with synthesis and testing to accelerate the process of identifying new
catalysts. This is a new development in a field that has historically
relied on trial and error.
“If
we can find new ways of rationally designing catalysts, we can speed up
the development of new catalytic materials enormously,” Nørskov said.
The
team first tackled the hydrogen half of the problem. The DTU
researchers created a device to harvest the energy from part of the
solar spectrum and used it to power the conversion of single hydrogen
ions into hydrogen gas. However, the process requires a catalyst to
facilitate the reaction. Platinum is already known as an efficient
catalyst, but platinum is too rare and too expensive for widespread use.
So the collaborators turned to nature for inspiration.
They investigated hydrogen producing enzymes—natural catalysts—from certain organisms, using a theoretical approach
Nørskov’s group has been developing to describe catalyst behavior. “We
did the calculations,” Nørskov explained, “and found out why these
enzymes work as well as they do.” These studies led them to related
compounds, which eventually took them to molybdenum sulfide. “Molybdenum
is an inexpensive solution” for catalyzing hydrogen production,
Chorkendorff said.
The
team also optimized parts of the device, introducing a “chemical solar
cell” designed to capture as much solar energy as possible. The
experimental researchers at DTU designed light absorbers that consist of
silicon arranged in closely packed pillars, and dotted the pillars with
tiny clusters of the molybdenum sulfide. When they exposed the pillars
to light, hydrogen gas bubbled up—as quickly as if they’d used costly
platinum.
The
hydrogen gas-generating device is only half of a full
photo-electrochemical cell. The other half of the PEC would generate
oxygen gas from the water; though hydrogen gas is the goal, without the
simultaneous generation of oxygen, the whole PEC cell shuts down. Many
groups—including Chorkendorff, Dahl and Nørskov and their colleagues—are
working on finding catalysts and sunlight absorbers to do this well.
“This is the most difficult half of the problem, and we are attacking
this in the same way as we attacked the hydrogen side,” Dahl said.
Nørskov
looks forward to solving that problem as well. “A sustainable energy
choice that no one can afford is not sustainable at all,” he said. “I
hope this approach will enable us to choose a truly sustainable fuel.”
