Hydrogen release from storage materials
Scientists at the University of California, Santa Barbara, have shed new light on the kinetics of hydrogen release.
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.
“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.”
Dr Lars Ismer and Anderson Janotti performed 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.
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 extracted key parameters from these calculations, and used them in Kinetic Monte Carlo simulations aimed at modeling how hydrogen is released, leaving clusters of aluminum atoms behind.
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.
“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 re-examined.”