Stability of magnesium-based nanoparticles for hydrogen storage

PhD ceremony: Mr. G. Krishnan, 14.45 uur, Academiegebouw, Broerstraat 5, Groningen

Title: Stability of magnesium-based nanoparticles for hydrogen storage

Promotor(s): prof. B.J. Kooi, prof. J.T.M. de Hosson

Faculty: Mathematics and Natural Sciences

The main motivation of this thesis was to analyze the structural details and the changes that can occur within the Magnesium nanoparticles during hydrogen absorption/desorption and heating treatments.

In the course of the investigation we were confronted with the problem associated with the thermal stability of magnesium nanoparticles. Mg nanoparticles with sizes in-between 10-50 nm showed the formation of voids and hollow Mg cores, i.e. only the (3 nm) MgO shell remains after heating at 300 degree celsius for a few hours. A crucial aspect is that size reduction is favoring the evaporation of the Mg, because a higher vapor pressure is associated with smaller nanoparticles.

Therefore: to overcome these limitations, different strategies have been investigated that can help to alleviate the thermal stability limitations of Mg nanoparticles (NPs).

(i) Addition of Cu prevents void formation during NP production and reduces the fast evaporation of Mg during annealing;

(ii) Alloying can prevent Mg evaporation: e.g. Mg with Ni forms a thermally stable core/shell (MgNi2/Ni) preventing Mg evaporation during annealing.

(iii) Covering Mg NPs with a Ti film leads to suppression of Mg evaporation during annealing.

In addition we also discovered how the presence of oxygen and the MgO shell around the Mg core is very crucial for the stability of the Mg NPs. The addition of Ti to the Mg target reduces the Mg-NPs size and leads to less protective Mg-oxide shells for NP <10 nm. This in return accelerates the evaporation of Mg that leads to hollow Mg cores.