Iowa — Hydrogen is being touted as the fuel
of the future, a clean-burning, renewable and inexpensive
replacement for petroleum. But a major stumbling
block for hydrogen-powered vehicles is figuring out
a way to carry enough hydrogen onboard to travel
even moderate distances between refueling stops.
Researchers at the U.S. Department of Energy's Ames Laboratory will be investigating
a possible solution to that problem thanks to $1.6 million in funding announced
recently by DOE Secretary Samuel Bodman as part of a $64 Million Hydrogen Fuel
“With compressed hydrogen gas, you simply can't carry a tank big enough to travel
very far,” Ames Lab senior scientist Vitalij Pecharsky said. “The answer is a
hydrogen-rich, solid fuel that mimicks the hydrogen content of methane, where
four hydrogen atoms encapsulate a single carbon atom.”
So why not just use methane? According to Pecharsky, methane and similar
hydrocarbon compounds have covalent bonds that keep the hydrogen atoms tightly “locked” in
place. The energy required to break those bonds is very high compared to the
energy you'd get from the hydrogen produced. Also, methane and other hydrocarbons
that come from oil are not renewable. The ideal solution would be a hydrogen-rich
solid material that would give up its hydrogen atoms easily, through moderate
heating or by other means. These materials could also be “recharged” – absorbing “new” hydrogen
atoms during refueling from a pressurized hydrogen gas source.
That's why Pecharsky and fellow Ames Laboratory scientists Marek Pruski,
Victor Lin and Scott Chumbley are looking at some novel materials – light-metal alanates,
borohydrides, amides, imides, and their derivatives – that have a total hydrogen
content exceeding 10 percent by weight.
A key component in the research project is solvent-free mechanochemical processing,
a technique Ames Laboratory researchers had shown back in 2002 to work well
when applied to complex hydrides. The process uses variable energy milling
to modify both the structure and properties of hydrides, and potentially, to
make them easily rechargeable with hydrogen. Materials to be processed are
placed in a hardened steel vial along with steel balls. The vial is vigorously
shaken and mechanical energy transferred into the system alters the crystallinity
of the solids and provides mass transfer, eventually breaking down the solids
and releasing hydrogen, or combining the materials and hydrogen gas into new
“Processing these materials without the use of solvents
is important,” Pecharsky said, “because once a material
is dissolved, its structure fundamentally changes.
Creating these complex hydride compounds in solid
state will allow us to look at the molecular structure
to see if there are ways to more easily get the hydrogen
back out of these systems.”
Another ingredient the group will use is called nanostructuring. Ames Lab chemist
Victor Lin has developed a way of using the nanoscale pores in a self-assembling
polymer as “molds” to precisely control the size of the material particles
going into the milling process. Smaller particles have higher surface energies
and surface energy may be a decisive factor in shifting thermodynamic equilibrium.
Lowering the size of particles to a few nanometers also reduces the distances
over which the mass transport takes place, thus improving the kinetics – the
rates of the reactions – of complex hydride-hydrogen systems.
Synthesizing various combinations and sizes of materials will provide samples
to be studied and characterized using a variety of high-tech methods. Ames
Lab scientist Scott Chumbley hopes that scanning and transmission electron
microscopy will give researchers a close-up look at the structure of the processed
materials. The team will also rely on the expertise of Ames Lab senior scientist
Marek Pruski in using solid-state nuclear magnetic resonance. Earlier studies
performed by Pruski's group proved that NMR is uniquely suited for the studies
of complex phases resulting from the milling process. Coupled with X-ray powder
diffraction, and other traditional materials characterization techniques, researchers
hope to gain a fundamental understanding of the relationships between the chemical
composition, bonding, structure, microstructure, properties and performance
of these materials.
“We'll look at the rates of absorption and desorption of hydrogen as well as
the cycling properties of these materials at various temperatures and pressures,” Pecharsky
said. “Furthermore, we plan to modify these nanoparticles with titanium and other
transition metal catalysts and perform a full array of characterization and hydrogenation-dehydrogenation
property tests on these metal-doped nanostructured hydrides.”
Parallel with the materials' characterization, the group will work with physicist
Purusottam Jena of Virginia Commonwealth University to develop first-principle
theoretical models based on the experimental data. Those models will then be
used to predict outcomes of further experiments. The predictions and actual
results will be compared to see if the theory holds or needs further modification.
Ultimately, the theoretical model will be used to help steer research toward
the most promising compounds.
Funding for the project will be spread over three years. Ames
Laboratory is operated for the Department
of Energy by Iowa State University .
The Lab conducts research into various areas of national concern, including
energy resources, high-speed computer design, environmental cleanup and restoration,
and the synthesis and study of new materials
Vitalij Pecharsky ,
Materials and Engineering Physics (515) 294-8220
Kerry Gibson , Public Affairs (515)