Scientists at the University of California, Santa Barbara (UCSB) performed simulation studies on the mechanism of hydrogen release or dehydrogenation of aluminum hydride, AlH3. The results of their study could challenge basic assumptions on the field of reaction kinetics.
Hydrogen is an efficient energy carrier. It powers fuel cells in electric cars. It can store energy generated from renewable sources like solar and wind power at times of low demand. It is also the simplest and most abundant element on earth.
The challenge in using this energy carrier is achieving high-energy storage density and efficient kinetics for dehydrogenation when necessary. Chris Van de Walle, head of the Computational Materials Group in UCSB, thinks the answer lies in aluminum hydride.
Aluminum hydride is a promising energy storage material. The movement of hydrogen atoms within the crystal lattice accounts for the energy storing capacity of the material.
Van de Walle believes AlH3 is ideal because the binding energy for hydrogen is low, this allows for a fast release rate. However, the kinetic barrier is high enough to prevent the release rate from being too fast.
Lars Ismer and Anderson Janotti, co-researchers of Van de Walle, performed cutting-edge, first-principles calculations in order to study how hydrogen atoms diffuse through aluminum hydride. They discovered that creation of hydrogen vacancies facilitated these movements.
Hydrogen vacancies are lattice defects that allow diffusion of atoms. If every atom is in place, there would be no movement. However, if an atom is missing, a neighboring atom could fill the vacancy.
From the sophisticated calculations, researchers obtained key parameters that were used in the Kinetic Monte Carlo Simulations. This software program can perform modeling studies of what is happening inside the AlH3 lattice.
The researchers said the simulations allow them to model realistic system sizes and time scales. They were able to monitor the nucleation and growth of the aluminum phase and the rate of dehydrogenation.
They were also able to determine the rate-limiting mechanism, which was the diffusion process. A surprising result since the dehydrogenation had an s-shaped reaction curve.
An s-shaped curve usually means the diffusion process cannot be the rate-limiting factor. However, Van de Walle argues that a re-examination of old assumptions, adapted before sophisticated and computational studies were developed, is crucial at this point. Their research is a proof that some accepted rule of thumbs may be outdated.
This study was published in the June 21 issue of the Journal of Physical chemistry.