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

Research highlights: → MgH2 + NbF5 and MgH2 + ZrF4 nanocomposites exhibit a good hydrogen sorption stability. → Better stability of MgH2 modified by ZrF4 than by NbF5 additive was observed. → MgF2 formation during cycling loading of MgH2 + NbF5 composite was observed. → The catalitycal influence of ZrF4 and NbF5 on MgH2 decomposition process was observed. - Abstract: In this work, new results on the microstructure and hydrogen storage capacity of MgH2 with ZrF4 and NbF5 after cyclic loading are presented. Commercial MgH2 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 deg. C. Better sorption stability was observed for MgH2 with ZrF4 than for MgH2/NbF5. Its microstructure consisted of an MgH2 matrix and stable nano-sized ZrF4 particles embedded in the 'core' structure of the particles. The outer layer of the particles was identified as MgH2.The gradual decrease in the hydrogen storage capacity while cyclic loading for this particular material is due to some stabilization of the fraction of MgH2/Mg with continues increase of grain size in the MgH2/Mg regions, from about 10 nm after ball milling to hundreds of nanometers after cycling. The stabilization process makes a fraction of MgH2/Mg inactive in the process of hydrogen desorption/absorption. In contrast, the MgH2/NbF5 sample after cyclic loading exhibited an MgH2/Mg matrix with some amount of MgF2 phase and nano-sized Nb-rich precipitates. The formation of the MgF2 phase is mainly responsible for the lost of hydrogen storage capacity of the MgH2/NbF5 sample while cyclic loading.