Hydrogen Storage Research for Fuel Cells Stimulated by New Material
Research on hydrogen-fueled cars may be one step closer to
application thanks to a new form of hydride discovered by scientists at
The material, lithium borohydride, is a promising energy storage
system: it contains 18 weight percents of hydrogen, which makes it
attractive for use in hydrogen-fueled cars. Its drawback is that it
only releases hydrogen at quite high temperatures (above 300C). The
team has found a new form of the compound that could possibly release
hydrogen in mild conditions. This discovery was completely unexpected
from the point of view of theoretical predictions.
|Crystal structure of the LiBH4 new phase.|
Automotive industry regards hydrogen as a perspective energy
carrier. If a good hydrogen storage material will be developed, the
petrol in cars can be replaced by clean hydrogen energy. Five kilograms
of hydrogen would take you as far as twenty liters of petrol. Today
there are several compounds of interest, which are known to either
store relatively large amounts of hydrogen or release it easily, but
none do both in a way suitable for practical application.
Researchers at the Swiss-Norwegian experimental stations
(beamlines) at the ESRF are currently studying several compounds of
light elements with hydrogen and the different forms they take at
different pressure and temperature. Lithium borohydride, LiBH4, is one
of the compounds they study as it has a high weight content of hydrogen
(18%). The new form of this compound, which scientists have just
discovered, is promising because it appears to be unstable. Until
today, all the known forms of this material are too stable, which means
that they don’t let the hydrogen go. “This one is
really unexpected and very encouraging”, says Yaroslav
Filinchuk, the corresponding author of the paper.
In order to obtain new forms of lithium borohydride, the team
applied to the sample pressures up to 200,000 bar. The pressure of
200,000 bar applied to LiBH4 in the ESRF experiment is about 80 times
bigger than the pressure exerted on Earth's crust by Mount Everest (the
latter is roughly equal to 2.5 kbar). Although impressive, this figure
is not a record - much higher pressures still can be reached in the lab
using the same diamond anvil cell technique, but this was not necessary
for this experiment.
Diffraction of synchrotron light was used to determine
arrangement of atoms in the resulting materials. In this way two novel
structures of lithium borohydride were found. One of them is truly
unprecedented (image 1) and reveals strikingly short contacts between
hydrogen atoms (image 2).
Combined experimental and theoretical efforts suggest that the
new from of LiBH4 can release hydrogen at a lower temperature.
Filinchuk explains that “the new form becomes even more
attractive considering the fact it appears already at 10.000 bar, the
pressure used by pharmaceutical companies to compress
pellets”. The authors argue that this form can be stabilized
by chemical substitutions even at ambient pressure. For now, the
team’s next step is to apply chemical engineering to the
compound to “freeze” the new form at ambient
conditions and check whether it shows more favorable hydrogen storage
properties than pure lithium borohydride.
Despite the fact that hydrogen is not well detected by X-rays
in general, scientists managed to see it thanks to the high brilliance
of the ESRF synchrotron light. Although theory failed to predict the
novel structure, it fully supports this experimental finding.
Therefore, this work presents a breakthrough in experimental studies of
hydrogen-rich system, explains the failure of the previous theoretical
predictions and suggests the novel form of the compound to be
instrumental in obtaining improved hydrogen storage materials.
Synchrotron radiation was recently successfully applied to
potential hydrogen storage materials and it turns out to be more useful
than generally expected for so light systems. The team at the
Swiss-Norwegian Beam Lines at the ESRF will continue to exploit and
develop this at first glance unexpected union.
Posted 5th December 2007