Hydrogen vacancy clustering in aluminum hydride (AlH3): The dark blue atoms represent the Al atoms in the regular AlH3 lattice, and the light blue atoms represent the Al atoms in the Al-rich region created by the clustering of hydrogen vacancies as hydrogen leaves the material. Credit: Van de Walle Group |
Hydrogen,
the simplest and most abundant element on Earth, is a promising energy
carrier for emerging clean energy technology. Hydrogen is the energy
carrier that powers fuel cells in electric cars, and can be used to
store energy generated by renewable sources at times of low demand.
A
major challenge with hydrogen energy is meeting the dual goals of high
storage density and efficient kinetics for hydrogen release when it is
needed.
Scientists
at the University of California, Santa Barbara, have shed new light on
the kinetics of hydrogen release, or dehydrogenation, from aluminum
hydride (AlH3), a material that is highly promising for energy storage.
Their computer simulations also illuminate the basic mechanisms
governing these chemical reactions in general.
“Aluminum
hydride turns out to be promising because the binding energy for
hydrogen is low, so that the release rate can be fast,” explained Chris
Van de Walle, a professor in the Materials Department and head of the
Computational Materials group at UCSB. “At the same time, kinetic
barriers are high enough to prevent the hydrogen release rate from being
too fast.”
Drs.
Lars Ismer and Anderson Janotti in the Computational Materials group
used computer simulations to investigate the microscopic mechanisms that
drive hydrogen release from aluminum hydride. They performed
cutting-edge, first-principles calculations to examine how individual
hydrogen atoms diffuse through the aluminum hydride—a process they found
to be enabled by the creation of hydrogen vacancies. Their findings
were detailed in a paper “Dehydrogenation of AlH3 via the Vacancy
Clustering Mechanism” published in The Journal of Physical Chemistry.
Hydrogen
vacancies are defects that play an important role—they enable
diffusion. If every atom is in place, none of the atoms would be able to
move. If a hydrogen atom is missing, a neighboring hydrogen atom can
jump into that vacancy, thus enabling motion of hydrogen through the
material.
The
group then extracted key parameters from these highly sophisticated
calculations, and used them in Kinetic Monte Carlo simulations aimed at
modeling how hydrogen is released, leaving clusters of aluminum atoms
behind.
“This
multi-scale approach allows us to take the highly accurate information
obtained in the first-principles computations and employ it to model
realistic system sizes and time scales,” said Ismer. “We can monitor the
nucleation and growth of the aluminum phase and the rate at which
hydrogen is released.”
An
important feature of the simulations is that they allowed the
researchers to identify the rate-limiting mechanism, which turned out to
be the diffusion process. This result initially seemed to contradict
conclusions from studies using the traditional interpretation of the
observed S-shaped onset of the dehydrogenation curves, which ruled out
diffusion as the rate-limiting factor. However, the UCSB team’s
simulations produced reaction curves in agreement with the measurements,
while clearly indicating that the reaction is limited by diffusion of
point defects.
“These
concepts transcend the specific application to hydrogen-storage
materials,” said Van de Walle. “The broader lesson here is that caution
should be exercised in drawing conclusions based solely on the shape of
reaction curves. Those simple rules of thumb were developed back in the
1930s, when experiments were less sophisticated and computational
studies were unheard of. Our present work strongly suggests that
traditional assumptions based on the shape of reaction curves should be
reexamined.”
Professor
Van de Walle’s Computational Materials Group is affiliated with the
Materials Department and the College of Engineering at UC Santa Barbara.
The group explores materials for hydrogen storage and generation,
complex oxides, nitride semiconductors, novel channel materials and
dielectrics, and materials for quantum computing.
This research was supported by the U.S. Department of Energy.
