The hydrogen storage performance and catalytic mechanisms of the MgH₂-ZrF₄ composite system

1. Introduction

Magnesium hydride (MgH₂) is regarded as a solid-state hydrogen storage material with high potential. Its avantages include low cost, excellent reversibility of release/uptake of hydrogen and abundance on earth compared to other solid-state hydrogen storage materials. Unfortunately, MgH₂ released hydrogen at very high temperatures (> 400 °C for the pure MgH₂), sluggishly absorbed and desorbed hydrogen, and stable thermodynamic properties restricted commercialization of MgH₂ for hydrogen storage applications. To overcome this problem, many approaches have been studied, such as using high-energy ball milling to synthesize nanoparticles of MgH₂ using catalysts or additives that lower the energy barrier for Mg-H bond breaking and shorten hydrogen diffusion pathways, and mixing with other metals or hydrides (destabilized concept) that will alter the thermodynamic properties of MgH₂.

2. Methods

This study examines the impact of zirconium fluoride (ZrF₄) addition on the MgH₂ synthesized using ball milling to improve hydrogen storage properties. The prepared 10 wt.% ZrF₄-doped MgH₂ sample was characterized by X-ray diffraction, pressure–composition–temperature (PCT) curves and differential scanning calorimetry (DSC).

3. Results

From the PCT characterization, the 10 wt.% ZrF₄-doped MgH₂ sample demonstrated improved hydrogen storage properties relative to as-milled MgH₂, with the onset decomposition temperature reduced from 340 °C to 220 °C. The absorption/desorption kinetics measurements proved that the sample of 10 wt.% ZrF₄-doped MgH₂ can release and uptake hydrogen much faster than the pure as-milled MgH₂. From the DSC measurement at different heating rates, the apparent activation energy for hydrogen release from 10 wt.% ZrF₄-doped MgH₂ is reduced significantly compared to the undoped MgH₂.

4. Conclusions

The improved hydrogen storage properties of MgH₂ in the presence of ZrF₄ are thought to be due to the function of in situ produced active species that form during the dehydrogenation process. This active species plays a critical role in catalyzing the hydrogen release and uptake in the ZrF₄-doped MgH₂ composite system.