New catalyst could cut cost of making hydrogen fuel

MADISON, Wis. - A discovery at the University of Wisconsin-Madison may

represent a significant advance in the quest to create a "hydrogen economy"
that would use this abundant element to store and transfer energy.

Theoretically, hydrogen is the ultimate non-carbon, non-polluting fuel for
storing intermittent energy from the wind or sun. When burned for energy,
hydrogen produces water but no carbon dioxide. Practically speaking,
producing hydrogen from water, and then storing and using the gas, have
proven difficult.

The new study, now published online at the Journal of the American Chemical
Society, introduces a new catalyst structure that can facilitate the use of
electricity to produce hydrogen gas from water.

Significantly, the catalyst avoids the rare, expensive metal platinum that
is normally required for this reaction. (Catalysts speed up chemical
reactions without themselves being consumed.)

The material under study, molybdenum disulfide, contains two common
elements, notes Mark Lukowski, a Ph.D. student working with associate
professor Song Jin in the UW-Madison chemistry department. "Most people have
tried to reduce the cost of the catalyst by making small particles that use
less platinum, but here we got rid of the platinum altogether and still got
reasonably high performance."

The research group has produced milligram quantities of the catalyst, "but
in principle you could scale this up," says Lukowski. "Molybdenum disulfide
is a commercially available product. To control purity and structure, we go
through the trouble of synthesizing it from the bottom up, but you could buy
it today."

To make the new material, Lukowski and Jin deposit nanostructures of
molybdenum disulfide on a disk of graphite and then apply a lithium
treatment to create a different structure with different properties.

Just as carbon can form diamond for jewelry and graphite for writing,
molybdenum disulfide can be a semiconductor or a metallic phase, depending
on structure. When the compound is grown on the graphite, it is a
semiconductor, but it becomes metallic after the lithium treatment. Lukowski
and Jin discovered that the metallic phase has far greater catalytic
properties.

"Like graphite, which is made up of a stack of sheets that easily separate,
molybdenum disulfide is made up of individual sheets that can come apart,
and previous studies have shown that the catalytically active sites are
located along the edges of the sheets," says Lukowski.

"The lithium treatment both causes the semiconducting-to-metallic phase
change and separates the sheets, creating more edges. We have taken away the
limitation from molybdenum disulfide and made the active sites both more
pervasive and more reactive."

The experiment, supported by the U.S. Department of Energy's Basic Energy
Sciences program, is a proof of concept for a new approach for improving
these catalysts, says Jin.

"Even though the efficiency of producing hydrogen has been greatly improved,
it is still not as good as what platinum can achieve," he says. "The next
steps include finding ways to further improve the performance by optimizing
all aspects of the process and exploring related compounds. There are many
hurdles to achieving a hydrogen economy, but the advantages in efficiency
and pollution reduction are so significant that we must push ahead."

As technological advances put further strain on the supply of platinum and
other rare elements, using common elements is a major advantage, Jin
stresses. "The elements we use are cheap and abundant in earth's crust, and
the raw material is already commercially available at low cost. Building on
this discovery and new understanding, we would like to further improve these
materials to achieve the efficient production of hydrogen without using
precious metals."