Hydrogen/Deuterium absorption and desorption in thin MgAl films
MgAl alloys have attracted interest for their favorable hydrogen storage properties for more than 20 years. This relatively simple system, available commercially in ingot form, is attractive due to a combination of the relative low material cost and the environmentally benign nature of the alloy. In general, most studies found MgAl alloys promising, however, the kinetics was still inadequate for the rapid low temperature desorption required of a commercially viable hydrogen storage material for automotive and portable hydrogen applications. Low temperature hydrogen absorption in MgAl thin films has been achieved with a single layer Pd, a bilayer Ti/Pd, and Fe(Ti)/Pd catalysts. However, appreciable hydrogen desorption is only possible at temperatures too high for practical applications.
We are studying thin films sa model systems because it is easy to modify the film structure in a well-defined way. Neutron Reflectometry (NR) is a powerful technique to study hydrogen/deuterium absorption and desorption in thin films because NR is very sensitive to hydrogen and deuterium and is capable of determining the hdyrogen profile in the film on a nanometer scale.
Optimum alloy composition
By using NR on Pd-capped Mg(1-x)Al(x) thin films we could determine that Mg0.7Al0.3 is the optimum composition with respect to desorption temperature (175 °C) and stored weight percent (4.1 wt.%).
H. Fritzsche, E. Poirier, J. Haagsma, C. Ophus, E. Luber, C. T. Harrower, and D. Mitlin
A systematic neutron reflectometry study on hydrogen absorption in thin Mg1-xAlx alloy films
H. Fritzsche, M. Saoudi, J. Haagsma, C. Ophus, E. Luber, C. T. Harrower, and D. Mitlin
Neutron reflectometry study of hydrogen desorption in destabilized MgAl alloy thin films
Interdiffusion of Mg and Pd
The Mg0.7Al0.3 films are fully desorbed at 448 K, whereas for the Mg0.6Al0.4 films a temperature of 473 K is needed to fully desorb the hydrogen. Our NR measurements show that the higher annealing temperature needed to desorb the hydrogen from the Mg0.6Al0.4 films led to an interdiffusion of the Pd layer into the MgAl layer. This Pd interdiffusion was also observed in a Mg0.7Al0.3 film after a 9 h annealing at 473K. So, the Pd interdiffusion into a MgAl film that has been charged with hydrogen is a common feature of the Pd/Mg0.7Al0.3 and Pd/Mg0.6Al0.4 alloy system. In contrast, for the as-prepared hydrogen-free Pd/Mg0.7Al0.3 film the Pd layer stays intact and only a small interdiffusion zone occurs at the Pd/MgAl interface.
H. Fritzsche, M. Saoudi, J. Haagsma, C. Ophus, C. T. Harrower, and D. Mitlin
Structural changes of thin MgAl films during hydrogen desorption
Effect of Ta/Pd catalyst layer on desorption
By using a nanoscale (5 nm Ta/5 nm Pd) bilayer catalyst we achieved remarkable desorption kinetics for thin films. Full hydrogen desorption occurred at 100 °C with a noticeable desorption even at room temperature. This is a significant improvement relative to the 175 °C needed to fully desorb an identical film with a single Pd layer acting as the catalyst. Neutron reflectometry confirmed that the Ta/Pd bilayer remained intact both after hydrogen absorption and following the hydrogen desorption.
H. Fritzsche, C. Ophus, C. T. Harrower, E. Luber, and D. Mitlin
Low temperature hydrogen desorption in MgAl thin films achieved by using a nanoscale Ta/Pd bilayer catalyst