Microstructure and hydrogen storage capacity of magnesium hydride with zirconium and niobium fluoride additives after cyclic loading

• Published in: Journal of alloys and compounds 2011-09, Vol.509, p.S616-S620
• Main Authors: Malka, I.E., Bystrzycki, J., Płociński, T., Czujko, T.
• Format: Article
• Language: English
• Subjects: Cyclic hydriding/dehydriding; Cyclic loads; Fatigue (materials); Hydrogen storage; Hydrogen storage capacity; Magnesium; Magnesium hydride; Metal halides; Microstructure; Nanomaterials; Nanostructure; Sorption
• ISSN: 0925-8388; 1873-4669
• DOI: 10.1016/j.jallcom.2010.10.122

▶ MgH 2 + NbF 5 and MgH 2 + ZrF 4 nanocomposites exhibit a good hydrogen sorption stability. ▶ Better stability of MgH 2 modified by ZrF 4 than by NbF 5 additive was observed. ▶ MgF 2 formation during cycling loading of MgH 2 + NbF 5 composite was observed. ▶ The catalitycal influence of ZrF 4 and NbF 5 on MgH 2 decomposition process was observed. In this work, new results on the microstructure and hydrogen storage capacity of MgH 2 with ZrF 4 and NbF 5 after cyclic loading are presented. Commercial MgH 2 powder was mixed with 7 wt.% metal halide powder and subsequently ball milled in an inert atmosphere. The microstructure of the powders was investigated with high-resolution SEM using BSE/STEM/EDS detectors. The thin samples were prepared by FIB. The materials exhibited good reversibility and hydrogen sorption stability. However, the hydrogen storage capacity decreased in both materials after prolonged cycling at 325 °C. Better sorption stability was observed for MgH 2 with ZrF 4 than for MgH 2/NbF 5. Its microstructure consisted of an MgH 2 matrix and stable nano-sized ZrF 4 particles embedded in the “core” structure of the particles. The outer layer of the particles was identified as MgH 2.The gradual decrease in the hydrogen storage capacity while cyclic loading for this particular material is due to some stabilization of the fraction of MgH 2/Mg with continues increase of grain size in the MgH 2/Mg regions, from about 10 nm after ball milling to hundreds of nanometers after cycling. The stabilization process makes a fraction of MgH 2/Mg inactive in the process of hydrogen desorption/absorption. In contrast, the MgH 2/NbF 5 sample after cyclic loading exhibited an MgH 2/Mg matrix with some amount of MgF 2 phase and nano-sized Nb-rich precipitates. The formation of the MgF 2 phase is mainly responsible for the lost of hydrogen storage capacity of the MgH 2/NbF 5 sample while cyclic loading.