Structural characterization and dehydrogenation behavior of Mg–5 at.%Nb nano-composite processed by reactive milling

Abstract

The utilization of Mg in hydrogen storage applications has been hindered by its high sorption temperature and slow kinetics. Both nanocrystallization and addition of transition metal catalysts demonstrated to improve the hydrogen sorption kinetics of Mg. In this scenario, high energy ball milling has shown to be a suitable way for processing Mg based nanostructured material, especially when milling is carried out under a hydrogen atmosphere (reactive milling, RM), which provides a nanostructured and hydrogenated final product. In the present work, we investigated the structural evolution of Mg, Nb and a mixture of Mg–5 at.%Nb processed by reactive milling by using the X-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques. The dehydrogenation behavior was investigated by differential scanning calorimetry (DSC) and the hydrogen content measured using a Leco determinator. It was found that Nb rapidly absorbs hydrogen, forming the NbH 0.9 phase. On the other hand, Mg was only partially transformed into β-MgH 2 and γ-MgH 2 phases. The amount of non-reacted Mg apparently does not vary when the milling time increases from 25 to 48 h. When Mg–5 at.%Nb mixture was milled for 48 h, almost all Mg and Nb were transformed into hydrides. Only a small amount of non-reacted Mg still remained in the sample. This result clearly shows the catalytic effect of Nb on the hydrogen sorption of Mg. Another remarkable feature observed is that Mg also contributed for the Nb hydrogenation as the NbH 2 phase was detected in the composite, which does not happen when pure Nb was milled in the same conditions. The structural evolution of the Mg–Nb composite is discussed based on the results obtained when the milling time is increased and comparing these results with those obtained for the pure elements. The DSC thermograms revealed that both Mg and Mg–Nb samples presented lower dehydrogenation temperatures when compared with the polycrystalline Mg. The Mg–Nb nanocomposite showed the lowest desorption temperature (270 °C) and fastest kinetics of hydrogen sorption.