(657b) Hydrogen Storage Using Mg-Mixed Metal Hydrides

Persistent urban air
pollution, demand for zero or low emission vehicles and the need to reduce
foreign oil imports increase the need for renewable energy concept. Hydrogen is
a promising material since it is an environmentally harmless energy carrier. The
lack of satisfactory hydrogen storage systems which are safe, cheap and simple
is one of the main problems for the transition to a hydrogen-based energy system.
Mg-based alloys show potential as hydrogen storage materials because of the
high gravimetric density of MgH₂ (7.6 wt. %), as well as its abundant
supply and low cost as a raw material. However, they exhibit high enthalpies of
formation, poor hydrogenation/dehydrogenation kinetics, poor charge/discharge
cycling stability and high temperature requirements. Also, MgO formation and a
high dissociation barrier of clean magnesium surfaces are other major problems with
Mg-based alloys. These problems could be solved by mixing Mg with another
chemical component, adding a small amount of a catalytically active metal on
the surface of the Mg clusters, or by using small particle sizes ^[1].
The addition of LaNi₅ to MgH₂ reduced the temperature of
hydrogen absorption, accelerated the kinetics at room temperature and yielded
higher hydrogen storage capacity at elevated temperatures ^[2]. Furthermore,
Barkhordarian et al showed that Nb₂O₅ is the most
effective catalyst for the hydrogen sorption reaction of Mg known so far ^[3].

In this work, Mg-LaNi₅
and Mg-Nb₂O₅ composite systems were studied and compared
to obtain a better understanding of hydrogen storage capacity, adsorption/desorption
rates and to observe the effect of different transition metals on hydrogenation
kinetics. In order to quantify the effect of particle size on hydrogen storage
capacity, different ball-milling times were utilized to prepare the samples
before testing. Hydrogen storage capacities of the materials are measured by
performing adsorption/desorption experiments at different temperatures and
generating pressure-composition isotherm data. The results of the experiments
are described and compared with previous data from the literature.

References

1       
David,
E., ?An overview of advanced materials for hydrogen storage?, Journal of
Material Processing Technology 162-163 (2005) 169-177.

2       
Au,
M., ?Hydrogen storage properties of magnesium based nanostructured composite
materials?, Materials Science and Engineering B 117 (2005) 37-44

3       
Barkhordarian
G. et al, ?Fast hydrogen sorption kinetics of nanocrystalline Mg using Nb₂O₅
as catalyst?, Scripta Materialia 49 (2003) 213-217