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