Diffraction studies provided the insights needed to understand key molecules in hydrogen storage. |
For
nearly a century, nobody knew how the little molecule that’s in the
middle of many of today’s hydrogen storage and release concepts was
organized. Thanks to an interdisciplinary team of scientists at Pacific
Northwest National Laboratory and Los Alamos National Laboratory, the
structure of this molecule, known as DADB, has been determined. And
DADB’s structure was exactly opposite of what was expected in more ways
than one.
“The
irony,” said Dr. Tom Autrey, the PNNL scientist who led the research,
“is that the structure could not be that complex.” The challenge was in
understanding how one structure, containing a pair of nitrogen and boron
atoms surrounded by only 12 hydrogen atoms, stretched and twisted in
the solid molecular crystal.
Running
cars on fossil fuels presents growing problems, economically,
politically, and environmentally. Replacing fossil fuels with hydrogen
and fuel cells is an attractive option. Determining the structure of
DADB, created at the initial stages when hydrogen is released from the
popular hydrogen storage material ammonia borane, allows scientists to
accurately model and predict complex, molecular reactions in the solid
state. Understanding the subtleties of the structure of DADB also
provides insights into developing new materials with the perfect
properties to store energy in chemical bonds for efficient fuel cell
operations.
The
team began by synthesizing the DADB using a new method they developed
that allowed the molecular crystal to slowly form at room temperature.
They used solid-state nuclear magnetic resonance (NMR) spectroscopy to
study the molecule. The NMR spectrum of the molecular crystal was
surprisingly different than the NMR spectrum of the molecular complex in
solution. The team felt that the hydrogen atoms in the molecular
crystal might be influencing the arrangement of atoms.
“Theoreticians
couldn’t accurately predict the structure, and experimentalists weren’t
getting all the information needed with NMR,” said Dr. Gregory
Schenter, a chemical theorist on the study. “So, we used neutron
diffraction to see the missing pieces. It took a while, but we got that
‘ah-ha’ moment.”
With the added diffraction data, they could arrange the atoms in a pattern that explained the results they’d seen.
“Mark
Bowden solved the 100-year-old puzzle,” said Autrey of his PNNL
colleague. “He showed how the molecule’s structure was affected by the
interactions with the neighboring molecules.”
This
research resulted in two different arrangements of borohydride ions
(BH4-) giving the molecule its unique twisted structure.
What’s
next? This work is part of a series of broader efforts at PNNL to
answer the fundamental questions around how to activate hydrogen for use
in catalytic reactions as well as energy storage in chemical bonds for
use in fuel cell applications. These fundamental studies are needed if
the United States is to develop novel methods to store energy from solar
and other intermittent clean energy sources.
The
work was done in DOE’s EMSL, a national scientific user facility at
PNNL, and the Manuel Lujan Jr. Center operated by Los Alamos National
Security LLC.
The
work was done by Mark Bowden, David J. Heldebrant, Abhi Karkamkar,
Gregory K. Schenter, and Tom Autrey of Pacific Northwest National
Laboratory along with Thomas Proffen of Lujan Neutron Scattering Center,
Los Alamos National Laboratory.
Reference:
Bowden M, DJ Heldebrant, A Karkamkar, T Proffen, GK Schenter, and T
Autrey. 2010. “The diammoniate of diborane: Crystal structure and
hydrogen release.” Chemical Communications 46, 8564-8566.
Diffraction studies provided the insights needed to understand key molecules in hydrogen storage.
